TITLE:   GCN CIRCULAR
NUMBER:  41689
SUBJECT: LIGO/Virgo/KAGRA S250904ae: Identification of a GW compact binary merger candidate
DATE:    25/09/04 04:40:51 GMT
FROM:    Trang Nguyen <trang.nguyen@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250904ae during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-04 03:33:07.667 UTC (GPS time: 1440992005.667). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], and PyCBC Live [6] analysis pipelines.

S250904ae is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250904ae

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (11.0, 22.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1021 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3330 +/- 883 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
 [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41638
SUBJECT: LIGO/Virgo/KAGRA S250901cb: Identification of a GW compact binary merger candidate
DATE:    25/09/01 19:47:05 GMT
FROM:    shoki.iwaguchi@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250901cb during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-09-01 18:59:41.965 UTC (GPS time: 1440788399.965). The candidate was found by the Aframe [1], cWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S250901cb is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-14 Hz, or about one in 1e6 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250901cb

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (11.0, 22.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 935 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(13h05m, -07d35m, 30.96d, 9.71d, 118.56d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1960 +/- 524 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982
 [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41656
SUBJECT: LIGO/Virgo/KAGRA S250901cb: Updated Sky localization
DATE:    25/09/02 10:06:38 GMT
FROM:    Michael J. Williams at University of Portsmouth <michael.williams@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250901cb (GCN Circular 41638). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250901cb

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 581 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2018 +/- 482 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41606
SUBJECT: LIGO/Virgo/KAGRA S250830bp: Identification of a GW compact binary merger candidate
DATE:    25/08/30 10:55:06 GMT
FROM:    Pedro Jeronimo Santos da Silva <pedro.jeronimo@unesp.br>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250830bp during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-30 10:24:18.852 UTC (GPS time: 1440584676.852). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], MLy [6], PyCBC Live [7], and SPIIR [8] analysis pipelines.

S250830bp is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250830bp

The classification of the GW signal, in order of descending probability, is BBH (84%), NSBH (16%), Terrestrial (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [9] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [9] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 16%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [10], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [11], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 11 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 429 +/- 100 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
 [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [6] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034
 [7] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [8] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [9] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [10] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982
 [11] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41607
SUBJECT: LIGO/Virgo/KAGRA S250830bp: Updated Sky localization, EM Bright Classification, and Source Classification
DATE:    25/08/30 12:55:43 GMT
FROM:    Colm Talbot at University of Chicago <talbotcolm@gmail.com>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250830bp (GCN Circular 41606). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250830bp

Assuming the candidate is astrophysical, the updated classification of the GW signal, in order of descending probability,
is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Based on posterior support from parameter estimation [1], under the assumption that the candidate S250830bp is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 4 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(21h43m, -77d36m, 1.36d, 0.88d, 15.49d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 427 +/- 69 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe

TITLE:   GCN CIRCULAR
NUMBER:  41608
SUBJECT: LIGO/Virgo/KAGRA S250830bp: NED Galaxies in the 4-Update Localization Volume
DATE:    25/08/30 13:49:57 GMT
FROM:    David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>

David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)

On behalf of the NASA/IPAC Extragalactic Database (NED) Team.

We spatially cross-matched the LVK S250830bp-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 72 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:

Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4                       
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4/20

The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.

|                  objname|            ra|           dec|objtype|    DistMpc|DistMpc_unc|       m_NUV|   m_NUV_unc|        m_Ks|    m_Ks_unc|        m_W1|    m_W1_unc|    P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|
|WISEA J215127.50-771123.7|     327.86462|     -77.18992|      G|     452.47|       0.65|        null|        null|      12.307|       0.126|      10.624|       0.006|1.78e-05|  2.49e-06|
|WISEA J214848.24-774454.9|     327.20101|     -77.74860|      G|     463.54|       null|        null|        null|      13.779|       0.215|      12.525|       0.010|5.02e-05|  1.28e-06|
|WISEA J214657.45-773648.1|     326.73941|     -77.61338|      G|     462.69|       null|        null|        null|      13.264|       0.192|      12.822|       0.014|5.09e-05|  9.83e-07|
|WISEA J215919.74-775235.8|     329.83229|     -77.87662|      G|     451.54|       0.65|        null|        null|      12.506|       0.114|      12.064|       0.008|1.84e-05|  6.79e-07|
|WISEA J213625.95-773427.9|     324.10814|     -77.57443|      G|     382.59|       null|        null|        null|      13.364|       0.175|      12.873|       0.009|5.20e-05|  6.48e-07|
|WISEA J214445.54-771938.0|     326.18978|     -77.32723|      G|     452.13|       null|      21.430|       0.312|      13.324|       0.160|      13.240|       0.012|4.93e-05|  6.16e-07|
|WISEA J213608.67-772811.0|     324.03613|     -77.46974|      G|     356.09|       null|        null|        null|      13.524|       0.185|      12.576|       0.014|4.08e-05|  5.79e-07|
|WISEA J215658.48-774037.0|     329.24368|     -77.67696|      G|     403.25|       null|        null|        null|      13.090|       0.145|      12.789|       0.009|3.68e-05|  5.55e-07|
|WISEA J213700.75-773426.0|     324.25313|     -77.57391|      G|     325.21|       0.65|      20.310|       0.217|      12.016|       0.075|      11.838|       0.006|2.20e-05|  5.14e-07|
|WISEA J213454.05-772745.8|     323.72522|     -77.46275|      G|     372.81|       null|        null|        null|      13.059|       0.143|      13.016|       0.016|4.75e-05|  4.92e-07|
|WISEA J215146.52-780557.3|     327.94386|     -78.09927|      G|     368.25|       0.65|        null|        null|      12.882|       0.135|      12.228|       0.010|2.20e-05|  4.63e-07|
|WISEA J213733.97-780150.6|     324.39156|     -78.03074|      G|     345.10|       null|      20.443|       0.108|      13.417|       0.204|      11.831|       0.007|1.74e-05|  4.63e-07|
|WISEA J212943.06-775542.2|     322.42945|     -77.92839|      G|     372.02|       0.65|        null|        null|      12.273|       0.114|      12.187|       0.009|1.85e-05|  4.12e-07|
|WISEA J213608.13-772733.1|     324.03389|     -77.45922|      G|     319.73|       0.65|      21.351|       0.351|      12.228|       0.076|      11.985|       0.007|2.01e-05|  3.96e-07|
|WISEA J212913.11-773431.7|     322.30466|     -77.57550|      G|     378.32|       0.65|      20.656|       0.234|      13.354|       0.165|      12.886|       0.013|2.92e-05|  3.50e-07|
|WISEA J213641.68-775334.3|     324.17371|     -77.89287|      G|     411.08|       null|      21.445|       0.179|      13.521|       0.179|      13.546|       0.029|4.09e-05|  3.18e-07|
|WISEA J213632.95-765253.9|     324.13731|     -76.88165|      G|     460.55|       0.65|        null|        null|      12.341|       0.134|      12.450|       0.009|1.07e-05|  2.86e-07|
|WISEA J213724.56-773623.7|     324.35235|     -77.60660|      G|     536.58|       null|        null|        null|      13.729|       0.212|      13.039|       0.022|1.35e-05|  2.84e-07|
|WISEA J215903.81-773020.5|     329.76589|     -77.50571|      G|     426.05|       null|        null|        null|      13.020|       0.144|      12.826|       0.010|1.74e-05|  2.83e-07|
|WISEA J212501.23-771251.5|     321.25514|     -77.21431|      G|     436.36|       null|        null|        null|      13.307|       0.143|      12.903|       0.015|1.69e-05|  2.66e-07|

Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250830bp sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). 

TITLE:   GCN CIRCULAR
NUMBER:  41609
SUBJECT: LIGO/Virgo/KAGRA S250830bp GLADEnet Completeness: Potential Host Galaxies in the 90% Credible Volume
DATE:    25/08/30 14:01:52 GMT
FROM:    Maria Brozzetti at Università degli Studi di Perugia  <marialisa.brozzetti@ligo.org>

M. L. Brozzetti (UniPG/INFN), G. Dálya (L2IT/EotvosU), G. Greco (INFN), M. Bawaj (UniPG/INFN), T. Matcovich (UniPG/INFN), S. Cutini (INFN) , R. De Pietri (UniPR/INFN), Marica Branchesi (GSSI)

On behalf of the GLADEnet Team.

We analyzed the completeness of the GLADE+ [1] catalog within the 90% credible localization volume of the S250830bp event from the 4-Update alert from the GCN Circular 41606.

The completeness value is 6.6e-1 in the B-band using the last released skymap : [Bilby.multiorder.fits,0](https://gracedb.ligo.org/api/superevents/S250830bp/files/Bilby.multiorder.fits,0), which means that the catalogue contains 66% of the total light in the B-band expected from galaxies in the localization volume. 

A total of 517 galaxies are identified within the 90% gravitational volume. The complete list of galaxies can be downloaded from the GLADEnet webpage [2] : [https://virgo.pg.infn.it/gladenet/catalogs/](https://virgo.pg.infn.it/gladenet/catalogs/)

GLADEnet allows for the interactive visualization of the 90% localization area and its intersection with regions of high extinction as defined in GLADE+. Furthermore, the first 1000 galaxies can be explored interactively, enabling users to filter galaxies based on their 3D probability density or their absolute B magnitude. The ligo.skymap cross-match method [3,4] is used to obtain the list of galaxies.

References:

[1]GLADE+: An Extended Galaxy Catalogue for Multimessenger Searches with Advanced Gravitational-wave Detectors G. Dálya et al. MNRAS, 514,1, pp.1403-1411, 2022 

[2] GLADEnet: A progressive web app for multi-messenger cosmology and electromagnetic follow-ups of gravitational-wave sources M. L. Brozzetti, G. Dálya, G. Greco, M. Bawaj, T. Matcovich, M. Branchesi, T. Boch, M. Baumann, S. Cutini, R. De Pietri et al. (4 more) A&A, 684, A44 (2024)

[3] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Lett., 829, L15. doi:10.3847/2041-8205/829/1/L15

[4] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Supp., 226, 10. doi:10.3847/0067-0049/226/1/10

TITLE:   GCN CIRCULAR
NUMBER:  41610
SUBJECT: LIGO/Virgo/KAGRA S250830bp: Coverage and upper limits from MAXI/GSC observations
DATE:    25/08/30 14:05:37 GMT
FROM:    Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>

T. Mihara, N. Kawai (RIKEN),
M. Nakajima, H. Negoro, K. Takagi (Nihon U.),
S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU)
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S250830bp at 2025-08-30 10:24:18.852 UTC (GCN 41606, 41607).

At the trigger time of S250830bp, the high-voltage of MAXI/GSC was on,
but the FOV was out of the 90% credible region of the Bilby skymap.
The credible region was observed about 3 minutes before the GW trigger time with GSC, and 
the first one-orbit (92 min) scan observation after the event covered 100%
of the 90% credible region from 11:52:34 to 11:53:19 UTC (T0+5296 to T0+5341 sec).

No significant new source was found in the region in those observations.
A typical 1-sigma averaged upper limit obtained in one scan observation
is 20 mCrab at 2-20 keV.

If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.

TITLE:   GCN CIRCULAR
NUMBER:  41640
SUBJECT: LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates
DATE:    25/09/01 20:32:52 GMT
FROM:    Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>

Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 2025-09-01 03:39:18 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed four fields centered on the following ICRS coordinates:
| RA | DEC |
|----------|----------|
| 324.165777 | -77.51123 |
| 327.955221 | -77.54517 |
| 323.965777 | -77.61123 |
| 327.755221 | -77.64517 |

These pointings cover the 90% localization region of candidate gravitational-wave event S250830bp.

All fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 21.8 in g, 21.8 in r, and 21.5 in i, and 20.7 in z.

We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.

Poor observing conditions during observing complicated positive identification of transient candidates. However, we report one candidate below with good detections in multiple bands. The candidate is separated by 5.58 arcseconds from the center of the apparent host galaxy LEDA 236249, which does not have a reported redshift.

| ID          | ATNAME     | RA             | DEC           | MAG_G     | MAG_G_ERR     | MAG_R     | MAG_R_ERR     | MAG_I     | MAG_I_ERR     | MAG_Z     | MAG_Z_ERR     |
|---------    |--------    |------------    |-----------    |-------    |-----------    |-------    |-----------    |-------    |-----------    |-------    |-----------    |
| DESGW_3336579 | AT2025wpk | 329.476163 | -77.41256 | 22.41 | 0.24 | 21.39 | 0.09 | 21.1 | 0.06 | 20.93 | 0.14 |

Further observations are ongoing and we encourage followup of the event region and the above candidate.

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41643
SUBJECT: LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates
DATE:    25/09/01 22:04:52 GMT
FROM:    xjh@andrew.cmu.edu

Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Brendan O’Connor (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), James Freeburn (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report:

DECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-01T03:39 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607).

We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS new transients within the LVK 99% CI area, and we report here select transients of high quality and interest:

First, one matched to a NED galaxy within the 90% volume (GCN 41608):

| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | NED separation |
| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- |
| T202509012137497m775946 | AT 2025wpv | 324.457179 |  -77.996131 | 2025-09-01 03:57:43.776 | 22.6 | 0.3 | 1.16 | 1.6”

Next, a transient of a known Gaia quasar (Gavras et al. 2023):

| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | Gaia separation |
| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- |
| A202509012159035m780432 | AT 2025wpq | 329.764660 |  -78.075678 | 2025-09-01 03:41:20.544 | 20.3 | 0.3 | 0.6 | 0.165”

Finally, we confirm the transient reported by McMahon et al. (GCN 41640):

| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |
| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- |
| T202509012157542m772445 | AT 2025wpk | 329.475736 |  -77.412522 | 2025-09-01 03:49:33.024 | 22.2 | 0.2 | 1.2 

Further analysis is underway.

We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.

TITLE:   GCN CIRCULAR
NUMBER:  41601
SUBJECT: LIGO/Virgo/KAGRA S250830m: Identification of a GW compact binary merger candidate
DATE:    25/08/30 04:40:26 GMT
FROM:    Jyotirmaya Mohanta at University of Tsukuba  <s2430161@u.tsukuba.ac.jp>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250830m during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2025-08-30 03:27:09.028 UTC (GPS time: 1440559647.028). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], PyCBC Live [6], and SPIIR [7] analysis pipelines.

S250830m is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.2e-08 Hz, or about one in 2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250830m

The classification of the GW signal, in order of descending probability, is BBH (98%), Terrestrial (2%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [9], distributed via GCN and SCiMMA notices about 22 seconds after the candidate event time.
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [10], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 3130 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3705 +/- 1112 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
 [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [9] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982
 [10] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41626
SUBJECT: LIGO/Virgo/KAGRA S250830m: Updated Sky localization
DATE:    25/09/01 08:29:41 GMT
FROM:    alexandresebastien.goettel@LIGO.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250830m (GCN Circular 41601). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250830m

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 3194 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3731 +/- 1474 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41571
SUBJECT: LIGO/Virgo/KAGRA S250827fo: Identification of a GW compact binary merger candidate
DATE:    25/08/27 23:51:40 GMT
FROM:    Jyotirmaya Mohanta at University of Tsukuba  <s2430161@u.tsukuba.ac.jp>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250827fo during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-27 22:49:40.311 UTC (GPS time: 1440370198.311). The candidate was found by the cWB BBH [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S250827fo is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.3e-11 Hz, or about one in 1e3 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250827fo

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Noise transients (glitches) were present in LIGO Livingston detector data within a second of the event time, which may affect the parameters or the significance of the candidate.

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 6%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 31 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 967 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1064 +/- 295 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [5] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41578
SUBJECT: LIGO/Virgo/KAGRA S250827fo: Updated Sky localization
DATE:    25/08/28 15:42:12 GMT
FROM:    alexandresebastien.goettel@LIGO.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250827fo (GCN Circular 41571). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250827fo

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 470 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1005 +/- 261 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41552
SUBJECT: LIGO/Virgo/KAGRA S250827l: Identification of a GW compact binary merger candidate
DATE:    25/08/27 02:45:38 GMT
FROM:    Jyotirmaya Mohanta at University of Tsukuba  <s2430161@u.tsukuba.ac.jp>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250827l during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-27 01:27:57.538 UTC (GPS time: 1440293295.538). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], MLy [5], PyCBC Live [6], and SPIIR [7] analysis pipelines.

S250827l is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250827l

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 32 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1145 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2132 +/- 580 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034
 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41562
SUBJECT: LIGO/Virgo/KAGRA S250827l: Updated Sky localization
DATE:    25/08/27 14:14:22 GMT
FROM:    Colm Talbot at University of Chicago <talbotcolm@gmail.com>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250827l (GCN Circular 41552). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250827l

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 615 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2464 +/- 700 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41404
SUBJECT: LIGO/Virgo/KAGRA S250818t: Identification of a GW compact binary merger candidate
DATE:    25/08/18 02:52:38 GMT
FROM:    Jiyoon Sun <jiyoon.sun.h@gmail.com>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250818t during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-18 02:08:58.392 UTC (GPS time: 1439518156.392). The candidate was found by the GstLAL [1], MBTA [2], and PyCBC Live [3] analysis pipelines.

S250818t is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.7e-11 Hz, or about one in 1e3 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250818t

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [4] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [4] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 2%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 32 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 377 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1311 +/- 350 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [3] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [4] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [5] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41415
SUBJECT: LIGO/Virgo/KAGRA S250818t: Updated Sky localization
DATE:    25/08/18 17:16:28 GMT
FROM:    lucy.thomas@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250818t (GCN Circular 41404). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250818t

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 319 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1209 +/- 290 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41410
SUBJECT: LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations
DATE:    25/08/18 09:38:35 GMT
FROM:    Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>

M. Nakajima, H. Negoro, K. Takagi (Nihon U.),
N. Kawai, T. Mihara, (RIKEN),
S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU)
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S250818k at 2025-08-18 01:20:06.030 UTC.

At the trigger time of S250818k, the high-voltage of MAXI/GSC was on.
The instantaneous field of view of GSC at the GW trigger time covered 1% of the 90% credible region
of the bayestar sky map, in which we found no significant new X-ray source.
The first one-orbit (92 min) scan observation with GSC after the event covered 78%
of the 90% credible region of the bayestar skymap from 01:20:06 to 02:51:15 UTC (T0+0 to T0+5469 sec).

No significant new source was found in the region in the one-orbit scan observation.
A typical 1-sigma averaged upper limit obtained in one scan observation
is 20 mCrab at 2-20 keV.

If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.

TITLE:   GCN CIRCULAR
NUMBER:  41414
SUBJECT: LIGO/Virgo/KAGRA S250818k: Candidates from the Zwicky Transient Facility
DATE:    25/08/18 17:01:30 GMT
FROM:    Robert David Stein at JSI <rdstein@umd.edu>

Robert Stein (JSI/UMD), Tomás Ahumada (Caltech) Mansi Kasliwal (Caltech), Theophile du Laz (Caltech), Utkarsh Pathak (IITB), Vishwajeet Swain (IITB), Anirudh Salgundi (UNC), Varun Bhalerao (IITB), Xander J. Hall (CMU) report,

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: 

We observed the localization region of the LVK trigger S250818k with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2025-08-18 04:02 UTC, approximately 2.7 hours after merger. We covered 25.2% (168.3 sq deg) of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 22 mag. 
 
The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019) , and removing candidates with history of variability prior to the merger time. 

We are left with 58 transient candidates, all lying within the 95.0% localization of the skymap. We perform additional vetting of these candidates, and identify those which appear to be hosted in galaxies at plausible redshifts for S250818k.

We highlight one of these candidates, ZTF25abjmnps/AT2025ulz.

+--------------------------------------------------------------------------------+
| ZTF Name     | IAU Name  | RA (deg)    | DEC (deg)   | Filter | Mag   | MagErr |
+--------------------------------------------------------------------------------+
| ZTF25abjmnps | AT2025ulz | 237.9757129 | +30.9023146 | r      | 21.29 | 0.13   |
+--------------------------------------------------------------------------------+

ZTF25abjmnps is in an elliptical galaxy with a Legacy Survey photometric redshift of z = 0.091 +/- 0.016, is therefore consistent with the estimated distance of S250818k. The lower 95th percentile limit of the photoz is 0.057, and at this distance S250818k would have an absolute g-band peak magnitude of M=-16.1. However, the higher redshifts would be more consistent with a supernova luminosity of M=-17. 

Forced photometry of this source reveals several detections in our data. The source appears to possibly be fading in g-band, but given the low SNR and short baseline it is difficult to constrain this.

There are also several recent non-detections, but all upper limits are shallower than the magnitude of the transient in our images. We therefore cannot confirm whether the source is young, or if it predates the merger.

We encourage observations of ZTF25abjmnps, to determine the nature of this source. 

Analysis of the remaining ZTF candidates is ongoing.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. GROWTH acknowledges the generous support of the NSF under PIRE Grant No 1545949. Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski (Duev et al. 2019). The GROWTH India Telescope (GIT, Kumar et al., 2022) is set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. Its operations are partially supported by funding from the IIT Bombay alumni batch of 1994. The Fritz and SkyPortal projects acknowledge the generous support of The Gordon and Betty Moore Foundation.

TITLE:   GCN CIRCULAR
NUMBER:  41421
SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz
DATE:    25/08/19 01:00:05 GMT
FROM:    Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>

Malte Busmann (LMU), Xander J. Hall (Carnegie Mellon U.), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-18T19:35:11 for 40 x 180 s. We took 30 observations in the giJ configuration and 10 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at

g = (21.25 +/- 0.03) AB mag,
r = (21.43 +/- 0.06) AB mag.

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41433
SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz
DATE:    25/08/20 01:24:57 GMT
FROM:    xjh@andrew.cmu.edu

Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-19T20:09:46 for 22 x 180 s. We took 11 observations in the giJ configuration and 11 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at

g = (22.08 +/- 0.09) AB mag,
r = (21.83 +/- 0.06) AB mag.

We note significant reddening and fading from our previous report (GCN 41421), with the g band dropping 0.83 AB mag in 24.6 hours and r band now being brighter than g.

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

Further analysis is underway.

We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41436
SUBJECT: LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
DATE:    25/08/20 05:23:52 GMT
FROM:    Mansi Kasliwal at Caltech <mansikasliwal@gmail.com>

Viraj Karambelkar (Caltech), Mansi M. Kasliwal (Caltech) and Xander J.  Hall (CMU) report on behalf of the larger ZTF and GROWTH collaborations

We observed ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) starting UTC 2025-08-19 06:42 with the LRIS spectrograph on Keck I. We derive a redshift of 0.0848 based on host galaxy emission lines - this is consistent with the 3D GW localization for S250818k. Subtracting host galaxy light, we see a blue continuum excess from the transient, and no tell-tale supernova-like features. The data in-hand thus far appears consistent with a kilonova. However, continued monitoring (such as Busmann et al. GCN 41421, Hall et al. GCN 41433) is encouraged to rule out any future re-brightening from a faint and fast supernova Type IIb (e.g., Barna et al. 2025, arXiv:2506.15900). Panchromatic follow-up is ongoing and encouraged.

TITLE:   GCN CIRCULAR
NUMBER:  41437
SUBJECT: LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz
DATE:    25/08/20 06:04:02 GMT
FROM:    gwangeon.seong@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration (LVK) report:

We identified the compact binary merger candidate S250818k during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-18 01:20:06.030 UTC (GPS time: 1439515224.030). The candidate was found by the GstLAL SSM [1] and PyCBC Live [2] analysis pipelines. Two LVK Preliminary GCN Notices were issued within 5 minutes after the candidate was identified.

Based on the analysis of gravitational-wave (GW) data alone, this candidate does not meet our criteria for a high-significance public alert as its false alarm rate is estimated by the online analysis to be 6.8e-08 Hz or about one in 5 months. However, we are issuing this Circular to confirm the properties of the GW candidate because of its potential association with the optical transient ZTF25abjmnps / AT 2025ulz (Stein et al, GCN 41414) which has been confirmed and reported to be fading and reddening rapidly (Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436). The GW candidate’s properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250818k

The classification of the GW candidate, in order of descending probability, is Terrestrial (71%), BNS (29%), NSBH (<1%), or BBH (<1%).
Initial data quality checks found that there is no evidence for presence of a glitch or other data quality issues.

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is >99%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99%. [3] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses, assuming the candidate is astrophysical in origin.

Three sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 28 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 2 days after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 786 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 259 +/- 74 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Hanna et al. (2024) arXiv 2412.10951
 [2] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41439
SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz
DATE:    25/08/20 09:52:22 GMT
FROM:    Matt Nicholl at Queens University Belfast <matt.nicholl@qub.ac.uk>

M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), S. J. Smartt, K.W. Smith, J. Gillanders, S. Srivastav, H. Stevance, F. Stoppa, J. Tweddle (Oxford), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), K. C. Chambers, M. E. Huber, A. S. B. Schultz, T. de Boer,  J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G, Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), T.-W. Chen (NCU), A. Rest (STScI), C. Stubbs (Harvard):

We report pre-discovery limits on the source AT2025ulz, reported by Stein et al. (GCN 41414) and possibly associated with the sub-threshold gravitational wave candidate S20250818k (The LIGO-Virgo-Kagra Collaboration, GCN 41437), obtained with the Pan-STARRS telescope system (Chambers et al., 2016, ArXiv e-prints, 1612.05560). The Pan-STARRS system comprises of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters. In our primary NASA mission for Near-Earth Object discovery, we scan the visible night sky North of -50 degrees declination to magnitude depths m~22, weather and Moon permitting.

Examining recent data obtained during normal Pan-STARRS survey coverage (see Fulton et al. 2025, MNRAS, 542, 541) at the location of AT2025ulz, we report the following pre-discovery upper limits and time since the S20250818k trigger (t-t0) 

Date       t-t0 (days)  Magnitude   Band
60904.233  -0.82        <19.7       y
60895.314  -9.7         <21.0       i

We note that the y-band non-detection was obtained only ~0.8 days before the GW signal. This indicates that there was no bright, red transient at this position less than a day prior.

The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4).

Further Pan-STARRS grizy imaging of this source, and coverage of the S20250818k skymap (northern banana) is underway in the i-band with PS1 and PS2 jointly. 

The discoveries from this program are a byproduct of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast enabled through the STFC grants ST/P000312/1 and ST/T000198/1. 

TITLE:   GCN CIRCULAR
NUMBER:  41440
SUBJECT: LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification
DATE:    25/08/20 10:12:16 GMT
FROM:    Soichiro Morisaki at U. of Tokyo <soichiro.morisaki@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250818k (GCN Circular 41437). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250818k

Based on posterior support from parameter estimation [1], under the assumption that the candidate S250818k is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is 80%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is 80%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 7%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is >99%.

The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin.

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 949 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 237 +/- 62 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe

TITLE:   GCN CIRCULAR
NUMBER:  41441
SUBJECT: LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations
DATE:    25/08/20 11:04:15 GMT
FROM:    Cuán de Barra at UCD <cuan.debarra@ucdconnect.ie>

For S250818k (GCN 41437, LVK Collaboration) and using the updated bayestar skymap, Fermi-GBM was observing 51.0% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S250818k . An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates.

Part of the LVK localization region is behind the Earth for Fermi, located at an RA=66.9, Dec=-25.2 with a radius of 68.0 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395, we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2):

Timescale  Soft   Normal   Hard 
------------------------------------
0.128 s: 1.5 	2.5 	4.3
1.024 s: 0.66   0.87    1.5
8.192 s: 0.22   0.29    0.45

Assuming the median luminosity distance of 259.4 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s):

Timescale  Soft     Normal   Hard
------------------------------------
0.128s: 0.018 0.029 0.081
1.024s: 0.008 0.010 0.028
8.192s: 0.003 0.003 0.008

TITLE:   GCN CIRCULAR
NUMBER:  41443
SUBJECT: LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits
DATE:    25/08/20 13:06:27 GMT
FROM:    Gaurav Waratkar at IIT Bombay <gauravwaratkar@iitb.ac.in>

G. Waratkar (IITB), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

We have carried a search [1] for X-ray candidates in Astrosat CZTI data in a 100 sec window around the trigger time of the event S250818k (UTC 2025-08-18 01:20:06; LVK Collaboration, GCN Circ. 41437). We use the Bilby.offline0.multiorder.fits,0 map (https://gracedb.ligo.org/api/superevents/S250818k/files/Bilby.offline0.multiorder.fits,0) for our analysis. 

CZTI is a coded aperture mask instrument that has considerable effective area for about 29% of the entire sky, but is also sensitive to brighter transients from the entire sky. At the time of the merger, Astrosat's nominal pointing is RA,DEC = 00:46:53.8, 42:11:36.4 (11.7242,42.1934), which is ~88 deg away from the maximum probability location, which severely reduces the effective area of CZTI. At the time of the merger event, the Earth-satellite-transient angle corresponding to maximum probability location is ~33 deg and hence is occulted by Earth in satellite's frame. In a time interval of 100 sec around the event, the region of the localisation map which is not occulted by Earth in the satellite's frame has a total probability of 0.59 (59%).

We note that the source AT 2025ulz (Stein et al., GCN Circ. 41414) was occulted behind Earth for AstroSat at the time of the merger event. 

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from three of the four independent, identical quadrants to look for coincident spikes in the count rates. Quadrant C being noisy was excluded from further analysis. Searches were undertaken by binning the data in 0.1s, 1s, and 10s respectively. Statistical fluctuations in background count rates were estimated by using data from 5 preceding orbits. We selected confidence levels such that the probability of a false trigger in a 100 sec window is 10^-3, after excluding one noisy quadrant. We do not find any evidence for any hard X-ray transient in this window, in the CZTI energy range of 20-200 keV.

We use a detailed mass model of the satellite to calculate the direction-dependent instrument response for points in the visible sky. We then assume the source is modelled as a power law with photon index alpha = -1, and convert our count rate upper limits to direction-dependent flux limits. We obtain the following upper limits for source flux in the 20-200 keV band by taking a probability weighted mean over the visible sky:

| Bin (s) | Flux Limit (ergs/cm^2/s) | Fluence Limit (ergs/cm^2) |
|---------|--------------------------|---------------------------|
| 0.1     | 1.47e-06                 | 1.47e-07                  |
| 1.0     | 3.78e-07                 | 3.78e-07                  |
| 10.0    | 8.00e-08                 | 8.00e-07                  |

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI EMGW detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=emgw

[1] Waratkar et al. ApJ 976, 123 (2024) doi:10.3847/1538-4357/ad84e6

TITLE:   GCN CIRCULAR
NUMBER:  41451
SUBJECT: LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz
DATE:    25/08/20 15:30:45 GMT
FROM:    S. Srivastav at Oxford <shubhamsrivastav@gmail.com>

S. Srivastav (Oxford), K. W. Smith, S. J. Smartt (Oxford/QUB), D. R. Young, M. Nicholl, M. D. Fulton, T. Moore, A. Aamer, C. R. Angus, M. McCollum, S. Sim, J. Weston, X. Sheng (QUB), L. Shingles (GSI/QUB), J. Sommer (LMU), J. Gillanders, H. Stevance, L. Rhodes, A. Andersson (Oxford), L. Denneau, J. Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), J. Anderson (ESO), A. Jordan, V. Suc (UAI, Obstech) A. Rest (STScI), T.-W. Chen (NCU), C. Stubbs (Harvard):

We report pre-discovery upper limits from ATLAS observations of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave event S250818k (GCN 41437).

ATLAS observed the field most recently before the GW event on MJD 60903.32, 1.74 days prior to the trigger. The observations were made as part of regular survey operations in the o-band. During the August 2025 lunation, we have been experimenting with a different cadence and longer exposures. The data on MJD 60903.32 were taken as 4 x 110 sec exposures (compared to 30 sec exposures in normal operations). These longer exposures provide a 3-sigma upper limit of o > 21.6 mag (AB) after stacking the 4 individual measurements. The discovery r-band mag of r = 21.29 +/- 0.13 (Stein et al., GCN 41414) implies a significant 3-sigma rise in 1.74 days, or a new transient appearing on that timescale. 

The Asteroid Terrestrial-impact Last Alert System [ATLAS] project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889,  STFC grants ST/Y001605/1, ST/X001253/1, the Royal Society and Schmidt Sciences. ATLAS-Teide is an IAC instrument included in the present “Strategic plan of the Canarian Observatories”, funded by the European Union  NextGenerationEU  EQC2021-007122-P and ICT2022-007828 projects. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy,  Queen's University Belfast, University of Oxford, the Space Telescope Science Institute, the South African Astronomical Observatory, The Millennium Institute of Astrophysics (MAS), Chile, and the Instituto de Astrofisica De Canarias.

TITLE:   GCN CIRCULAR
NUMBER:  41452
SUBJECT: LIGO/Virgo/KAGRA S250818k: Multi-band Gemini GMOS Detections
DATE:    25/08/20 17:28:14 GMT
FROM:    Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>

B. O’Connor (CMU), J. Freeburn (UNC), X. J. Hall (CMU), Malte Busmann (LMU), I. Andreoni (UNC), A. Palmese (CMU), D. Gruen (LMU), L. Hu (CMU), T. Cabrera (CMU), K. Kunnumkai (CMU), A. Amsellem (CMU) report on behalf of a larger collaboration: 

Using Gemini GMOS-N, we observed the source AT 2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414) discovered during ZTF follow-up of the low-significance gravitational wave event S250818k (GCN 41437, 41440; https://gracedb.ligo.org/superevents/S250818k), which is a candidate binary neutron star merger. 

Following the discovery of rapid fading and reddening of the source with the Fraunhofer Telescope at Wendelstein Observatory (FTW; see Busmann et al. GCN 41421, Hall et al. GCN 41433), we carried out observations in the grizY filters with GMOS-N under photometric observing conditions starting at 2025-08-20 05:29:56 UT. This corresponds to 2.17 days after the gravitational wave detection (GCN 41437). 

A visual inspection reveals that the transient is significantly detected on top of the host galaxy. The source is clearly off-nuclear, but lies on top of the galaxy’s disk. We performed difference imaging with respect to Legacy Survey templates that further reveal the source. The brightness of the transient is g~23.0 AB mag and r~22.6 AB mag. The photometry is not correct for Galactic extinction. 

Our data reveals continued rapid fading and reddening of the transient in the 9.4 hours following the FTW report (Hall et al. GCN 41433). This would imply a steepening of the fade rate in g-band. We caution that the galaxy appears over subtracted in our difference imaging, which may affect the preliminary photometry. Additional analysis is underway. 

Further observations are planned and additional follow-up is encouraged.

We thank the staff of the Gemini Observatory, in particular Jen Andrews, Brian Lemaux, and Jen Miller, for their excellent support in rapidly scheduling these observations. 

TITLE:   GCN CIRCULAR
NUMBER:  41453
SUBJECT: LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz
DATE:    25/08/20 17:43:21 GMT
FROM:    xjh@andrew.cmu.edu

Xander J. Hall (Carnegie Mellon U.), Robert Stein (UMD), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We requested ToO observations of AT 2025ulz, reported by Stein et al. (GCN 41414) as a possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440).

The observation was taken with a midpoint time of 2025-08-19T11:48:55.755, approximately 30 hours after the trigger, with a duration of 3.5ks.

The source was not detected with XRT with an upper limit of 3.8 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.0848 (GCN 41436), this gives a rough luminosity upper limit of 3.3 * 10^42 erg/s. 

We clearly detect a source at the position of AT2025ulz in UVOT observations, conducted in the UVM2 filter, with a magnitude of m=22 AB. However, this flux is consistent with archival detections of the host galaxy in GALEX. We therefore find no evidence of transient UV emission in UVOT data.

We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible.

TITLE:   GCN CIRCULAR
NUMBER:  41454
SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz
DATE:    25/08/20 18:11:46 GMT
FROM:    James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>

J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard).

We observed the optical transient AT2025ulz (Stein et al., GCN 41414), the transient plausibly associated with the (sub-threshold) gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). 

Our observation consisted of 6x150s exposures in each of the grizy filters in both telescopes. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4).

From these difference images, we measure the following preliminary AB magnitudes, from averages of all forced photometry from the two GPC1 and GPC2 cameras:

MJD         t-t0 (days)    Filter    AB mag
60907.30    2.25           g         22.9 +/- 0.4
60907.31    2.26           r         22.2 +/- 0.2
60907.33    2.27           i         21.8 +/- 0.2
60907.34    2.28           z         21.4 +/- 0.2

Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437).

Our difference imaging and photometric measurements indicate a consistent flux excess at the position of AT2025ulz (Stein et al., GCN 41414), although we note that these current magnitude estimates are derived from difference images generated with reference images shallower than the target images (900s vs. ~300s, respectively). We caution these are preliminary, given the differencing issues and the closeness of the source to the bright nucleus and body of the host. 

Comparing our current Pan-STARRS gr-band data to that previously reported by Busmann et al. (GCN 41421) and Hall et al. (GCN 41433), we find that AT2025ulz appears to have faded further, if both sets of photometry are accurate. 

We can certainly conclude that no re-brightening (which may be expected in SNe IIb or shock cooling) is yet taking place. 

Work is ongoing to generate a deeper reference image, and thus improved magnitude estimates. Further multi-band observations are scheduled.

TITLE:   GCN CIRCULAR
NUMBER:  41456
SUBJECT: LIGO/Virgo/KAGRA S250818k: WINTER J-band observations of AT 2025ulz
DATE:    25/08/20 18:41:15 GMT
FROM:    Geoffrey Mo at MIT <gmo@mit.edu>

Geoffrey Mo (MIT), Robert Stein (JSI), Mansi Kasliwal (Caltech), Viraj Karambelkar (Caltech), Danielle Frostig (CfA), Nathan Lourie (MIT), and Robert Simcoe (MIT) report:

We observed AT2025ulz (Stein et al., GCN 41414), a possible counterpart to the LVK sub-threshold event S250818k (LVK, GCN 41437, 41440), with the 1.2 sq. degree near-IR WINTER camera on the Palomar 1-m telescope (Lourie et al. 2021, Frostig et al. 2024).

Our observations were conducted in J-band beginning at 2025-08-19 03:09:09 UTC, approximately 26 hours after the merger. Our observations consisted of 30 individual 120s dithers. The images were processed using the WINTER data reduction pipeline (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565) using images from the UKIRT Hemisphere survey (Dye et al. 2018) as references for image subtraction.

We do not detect any source at the position of AT2025ulz in our difference image. We place a J-band upper limit of m > 19.3 AB mag.

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

TITLE:   GCN CIRCULAR
NUMBER:  41460
SUBJECT: LIGO/Virgo/KAGRA S250818k: EP/FXT observation of AT 2025ulz
DATE:    25/08/21 08:38:43 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

R.-Z. Li (YNAO, CAS), X.-P. Xu, H. Sun, D.-Y. Li, Y. Liu, W. Yuan (NAO ,CAS) and B. Zhang (HKU) on behalf of the Einstein Probe (EP) team:

We performed a ToO observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) for AT2025ulz, reported by Stein et al. (GCN 41414) as a possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440).

The observation was carried out from 2025-08-20 08:31:13 (UTC) to 2025-08-20 13:19:05 (UTC), with a total exposure time of about 9 ks. The midpoint time of this observation is 2025-08-20T10:55:09 (UTC), approximately 57.6 hours after the trigger (LVK Collaboration, GCN 41437).

The source was not detected with EP/FXT with an upper limit of 1.1e-3 cts/s at 0.5-10 keV. The estimated flux upper limit is 1.68e-14 erg/cm^2/s (90% C.L., at 0.5-10 keV). Given the redshift of z = 0.0848 (GCN 41436), this gives a rough luminosity upper limit of 3.2e41 erg/s.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

TITLE:   GCN CIRCULAR
NUMBER:  41461
SUBJECT: LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz
DATE:    25/08/21 09:00:21 GMT
FROM:    Zhengyan Liu at USTC <ustclzy@mail.ustc.edu.cn>

Z. Y. Liu, Z. L. Xu, D. Z. Meng, J.-A. Jiang, W. Zhao, M. X. Cai, Z. G. Dai, L. L. Fan, N. Jiang, X. Kong, T. G. Wang (USTC), Y. Z. Fan, J. J. Geng, Z. P. Jin, X. F. Wu (PMO) report on behalf of the WFST Collaboration:
 
We report the pre-discovery upper limits and follow-up photometry results of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave (GW) event S250818k (GCN 41437), using the Wide Field Survey Telescope (WFST Collaboration; Wang et al., 2023).
 
After checking WFST archive data, we found u- and g-band 30s exposures taken at MJD 60901.66, about 3.4 days prior to the GW event trigger. The images were processed with the WFST pipeline, using stacked WFST multi-exposures taken in February 2025 as templates. We do not found any residuals in difference images in the two bands. We report the following pre-discovery 3-sigma upper limits and time since the S20250818k trigger (t-t0):
 
MJD t-t0 (days) AB mag Band
60901.64 -3.42 > 22.1 u
60901.67 -3.38 > 22.4 g
 
We conducted WFST follow-up observations of AT2025ulz with 120s exposures in g and r bands and a 180s exposure in u band beginning at 2025-08-20T14:24:03 UTC. After image subtraction, a clear source was detected in g band, and faint sources detected in u and r bands, which are close to the detection limit. The preliminary AB magnitudes of AT2025ulz are (all photometries are not corrected for Galactic extinction):
 
MJD t-t0 (days) AB mag Band
60907.60 2.54 22.71+/-0.10 g
60907.61 2.55 22.78+/-0.16 r
60907.65 2.59 23.24+/-0.24 u
 
The g-r and u-g color from WFST observations are approximately -0.1 mag and 0.5 mag, respectively, which became bluer compared with the previous reports (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454). Additionally, our results show a re-brightening trend for AT2025ulz, which may be related to the shock-cooling process mentioned in Gillanders et al. (GCN 41454). We also note that AT2025ulz is significantly more luminous than AT 2017gfo at the same phase, especially in g and u bands assuming a host redshift 0.0848 reported by Karambelkar et al. (GCN 41436). The angular separations between the detected source in g band and the host center are approximately 0.85 and 0.83 arcseconds using host coordinates in PS1 DR2 (Chambers et al., 2016) and SDSS DR16 (Ahumada et al., 2020), respectively. Given the close distance between the source and the bright host nucleus, and the peculiar photometric evolution in the past 24 hours, we suggest that the variability might be influenced by the host-nuclear activity. Further deep multiband imaging follow-ups are highly encouraged.

TITLE:   GCN CIRCULAR
NUMBER:  41464
SUBJECT: LIGO/Virgo/KAGRA S250818k: 10 GHz VLA observations of AT2025ulz
DATE:    25/08/21 12:54:35 GMT
FROM:    Roberto Ricci at INAF-IRA <ricci@ira.inaf.it>

Authors: R. Ricci, M. Yadav, E. Troja (U Rome) report:

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) possibly associated to the GW candidate S250818k (LVKC GCN 41437) with the Very Large Array in X-band at the centre frequency of 10 GHz with a bandwidth of 4 GHz on Aug 21st 2025, 3.1 days after the GW trigger. 

After reducing the data using the latest version of the CASA VLA pipeline (v6.6.1) and standard imaging procedures, we found no detection at the optical transient position down to a 3-sigma flux density upper limit of 30 microJy/beam. 

We thank the VLA staff for promptly executing the observations.

TITLE:   GCN CIRCULAR
NUMBER:  41468
SUBJECT: LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits
DATE:    25/08/21 16:29:36 GMT
FROM:    kendall.ackley@warwick.ac.uk

K. Ackley, A. Kumar, D. O'Neill, M. J. Dyer, J. Lyman, K. Ulaczyk, S. Belkin, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, D. Pollacco, J. Casares Velazquez, T. Killestein report on behalf of the GOTO collaboration:

We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al. 2024) in response to the LIGO/Virgo/KAGRA low-significance event S250818k (The LIGO-Virgo-KAGRA Collaboration, GCN 41437). 

As part of our strategy for LVK O4 follow-up, targeted observations of the event between 2025-08-18 01:22:29 UT (2.4 minutes post-trigger) and 2025-08-21 05:29:32 UT (76.2 hours post trigger) with GOTO-North and between 2025-08-18 08:34:23 UT (7.2 hours post-trigger) and 2025-08-19 09:29:09 UT (32.2 hours post-trigger) with GOTO-South.

We covered a total of 521.4 deg2 (46.0%) of the localisation probability of the Bilby skymap (The LIGO-Virgo-KAGRA Collaboration, GCN 41440). Exposures were taken in L-band (400-700 nm passband) and the average 5 sigma depth was L = 19.95 AB magnitudes with GOTO-1 (North), L = 19.74 AB with GOTO-2 (North), L = 19.05 AB with GOTO-3 (South), and L = 19.70 AB with GOTO-4 (South).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. 

No significant counterpart sources that can be associated with S250818k have been identified. We do not detect emission of the candidate counterpart AT 2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) down to a 3-sigma limiting magnitude of L = 20.3 AB at 2025-08-18 20:53:41 UTC (~19.6 hours post-trigger).

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

TITLE:   GCN CIRCULAR
NUMBER:  41474
SUBJECT: LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits
DATE:    25/08/21 19:06:59 GMT
FROM:    Sylvio Klose at TLS Tautenburg <klose@tls-tautenburg.de>


S. Klose, B. Stecklum, A. Nicuesa Guelbenzu, and F. Ludwig (all
Tautenburg) report:

We observed the optical transient ZTF25abjmnps/AT2025ulz (Stein et
al., GCN 41414), possibly associated with the gravitational wave event
S250818k (GCN 41437), with the Tautenburg 1.34-m Schmidt telescope
using the Sloan filter set.

Observations were performed on 20-08-2025 between 20:06 and 20:39 UT
as well as between 21:57 and 22:30 UT. They reached a 3-sigma upper
limit in r and i of 22.3 and 22.4 mag (AB), respectively. Photometric
calibration was performed using Pan-STARRS DR2.

At the position of the optical transient (OT) discovered by Stein et
al. (GCN 41414; see also Busmann et al., GCN 41421; Hall et al., GCN
41433), no point source superimposed the underlying host galaxy was
detected / could be identified.

Given the closeness of the OT to the host galaxy's nucleus (Liu et
al., GCN 41461) we caution that the photometric limits are preliminary.

TITLE:   GCN CIRCULAR
NUMBER:  41476
SUBJECT: LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of AT 2025ulz 
DATE:    25/08/21 20:58:24 GMT
FROM:    Andrew Levan at Radboud University <a.levan@astro.ru.nl>

Smaranika Banerjee (Stockholm University), Maria-Teresa Botticella (INAF - Capodimonte Obs.), Seán J. Brennan (MPE), Enrico Cappellaro (Padua Obs.), Ting-Wan Chen (NCU Taiwan), Paolo D'Avanzo (INAF - Brera Obs.), Max De Pasquale (Univ. Messina), Rob A. J. Eyles-Ferris (Leicester), Morgan Fraser (UCD), James H. Gillanders (Oxford), Ben Gompertz (Birmingham), Nusrin Habeeb (Leicester), Luca Izzo (INAF/OAC and DARK/NBI), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud and Warwick), Daniele Bjørn Malesani  (DAWN/NBI and Radboud), Antonio Martin-Carrillo (UCD), Sam Oates (Lancaster U.), Matt Nicholl (QUB), Andrea Rossi (INAF - Bologna), Om Sharan Salafia (INAF - Brera Obs.), Nikhil Sarin (Cambridge), Steve Schulze (Northwestern), Stephen J. Smartt (Oxford), Danny Steeghs (Warwick), Nial R. Tanvir (Leicester), Aishwarya L. Thakur (INAF-IAPS) report for the ENGRAVE collaboration:

“We report preliminary analysis of ESO VLT observations obtained by the ENGRAVE collaboration of AT 2025ulz (Stein et al., GCN 41414), which was discovered within the 3D localization of the GW alert S250818k (LVK Collaboration, GCNs 41437, 41440).

Following the announcement of the discovery of AT 2025ulz, we triggered optical imaging and spectroscopy, as well as NIR imaging using FORS2, X-shooter and HAWK-I.

Using X-shooter, we obtained total exposure times of 3480, 3600, 3600 s in the UVB, VIS and NIR arms respectively, between 2025 Aug 20.99 and Aug 21.04 UT. As AT 2025ulz is faint, we acquired the target using a blind offset. We see no obvious broad features in our extracted spectrum that would be suggestive of either a KN or a SN. We do, however, see galaxy features, with prominent lines from the Balmer series (alpha, beta, gamma and delta), Paschen-alpha, [OII], [OIII], [OI], [NII], [SII] in emission, and Ca II in absorption, which securely place the redshift at z = 0.0848 (consistent with Karambelkar et al., GCN 41436).

With HAWK-I we obtained 690 s of Ks-band imaging between Aug 20.96 and Aug 20.98 UT. While the host galaxy of AT 2025ulz is well detected, we see no obvious point source corresponding to the transient itself. We note that, in the optical, template subtraction is required to detect AT 2025ulz, and, as we have no deep templates, we are currently unable to do this in the NIR. We estimate a limiting magnitude of K > 23 (AB mag) for the image, and K > 22 at the location of AT 2025ulz.

We obtained 3000 s of FORS2 long-slit spectroscopy using the 300I grism at the position of AT 2025ulz between Aug 21.01 and Aug 21.05 UT, but again see no obvious signal from the transient. We also examined the 3x60 s I-band FORS2 acquisition images, but see no clear point source.

With the data in hand, we cannot confirm a signal that is kilonova-like. Neither of the two VLT spectra show signatures of a transient source and there is no detectable K-band emission to around -16 (AB mag) absolute magnitude. The host galaxy contamination makes faint object observations challenging without deep templates.

If AT 2025ulz is associated with a KN, then preliminary modeling of the optical fluxes reported (Busman et al. GCN 41421, Hall et al. GCN 41433, O'Connor et al. GCN 41452, Hall et al. GCN 41453, Gillanders et al. GCN 41454, Mo et al. GCN 41456, Lui et al. GCN 45461) indicate that it must have a fairly large ejecta mass (of perhaps ~0.05 Msun) and the lack of K-band detection indicates it may be lanthanide-poor. If AT 2025ulz is a cooling tail following shock breakout in a core-collapse supernova (as seen for example in SN 1993J), then we should expect the lightcurve to begin to rise again approximately four days from now. 

Further ENGRAVE observations of AT 2025ulz will be carried out at the VLT.

We acknowledge Célia Desgrange, Lorena Faundez, Camila de Sa Freitas, Elisa Garro, Cecilia Bustos, Claudia Cid, Miguel Lopez, Francesca Lucertini and the observing staff in Paranal for their excellent support.“

TITLE:   GCN CIRCULAR
NUMBER:  41480
SUBJECT: LIGO/Virgo/KAGRA S250818k: Continued fading and reddening of AT2025ulz from Keck/LRIS imaging observations
DATE:    25/08/21 23:06:59 GMT
FROM:    Daniel Perley at Liverpool JMU <d.a.perley@ljmu.ac.uk>

Daniel A. Perley (LJMU), Mansi M. Kasliwal (Caltech), Viraj Karambelkar (Caltech), Michael Lundquist (WMKO), Xander Hall (CMU), Tomas Ahumada (Caltech), Kritti Sharma (Caltech), and Sam Rose (Caltech) report on behalf of the larger ZTF and GROWTH collaboration:
 
The location of AT2025ulz (ZTF25abjmnps; Stein et al., GCN 41414), a candidate counterpart to S250818k (LVK Collaboration, GCNs 41437, 41440), was imaged using the Low-Resolution Imaging Spectrometer (LRIS) on the Keck I 10m Telescope on the consecutive nights of 2025 August 20 and 21 (UT), simultaneously in g-band and i-band on each night. On the first night weather conditions were good, and two exposures of 30s each were acquired in each band starting at 2025-08-20 07:04 UT.  The weather was cloudy on the second night.  Three 300s exposures were taken, but due to highly variable atmospheric extinction only the highest-transmission image (starting at 2025-08-21 07:45 UT) was used for this analysis.  The 5-sigma limiting magnitudes of the images are g>24.8, i>23.9 (08-20) and g>23.9, i>23.2 (08-21).

While determining the magnitude of the transient is limited by the depth of available reference images, we performed difference imaging between the two nights in each band. Subtracting the 08-20 image from the 08-21 image in each filter, there is a clear detection of a negative point source at the reported location of the transient in the g-band difference image, but no significant residual flux in the i-band difference image.   The difference flux at the transient position from forced photometry is -1.60 +/- 0.20 microJy (g-band) and +0.31 +/- 0.38 microJy (i-band).  This suggests the transient is still rapidly fading in g-band, and likely reddening in g-i color.  This observation is consistent with the kilonova hypothesis and inconsistent with the re-brightening proposed by Liu et al. (GCN 41461).

TITLE:   GCN CIRCULAR
NUMBER:  41489
SUBJECT: LIGO/Virgo/KAGRA S250818k: GRAWITA TNG NIR observations of AT2025ulz
DATE:    25/08/22 08:32:41 GMT
FROM:    Paolo D'Avanzo at INAF - OAB <paolo.davanzo@inaf.it>

P. D'Avanzo, (INAF-OAB), M.T. Botticella (INAF - OACn),  E. Cappellaro (INAF-OAPd), L. Izzo (INAF - OACn), A. Rossi (INAF-OAS), V. D’Elia (ASI-SSDC), S. Piranomonte (INAF-OAR), G. Greco (INFN), M.G. Bernardini (INAF-OAB), S. Ronchini (GSSI), B. Patricelli (Univ. Pisa), O. S. Salafia, G. Ghirlanda (INAF-OAB), N. Elias-Rosa (INAF-OAPd), Fabio Ragosta (Università degli studi di Napoli “Federico II”), R. Salvaterra (INAF-IASF Milano), E. Brocato (INAF-OAR), V. Lorenzi, A. Garcia De Gurtubai Escudero (INAF-TNG), on behalf of GRAWITA report:

We observed the optical transient AT2025ulz (Stein et al., GCN Circ. 41414; Busmann et al., GCN Circ. 41421; Hall et al., GCN Circ. 41433; Karambelkar et al., GCN Circ. 41436; O’Connor et al., GCN Circ. 41452; Gillanders et al., GCN Circ. 41454; Liu et al., GCN Circ. 41461; Perley et al., GCN Circ. 41480) possibly connected to the GW candidate event S280818k (LVK collaboration, GCN Circ. 41437 & GCN Circ. 41440) with the Italian 3.6m TNG telescope, located in Canary Islands (Spain), equipped with the near-infrared camera NICS in imaging mode. 
A series of images were obtained with the J filter starting on 2025-08-20T20:27:00 UT (i.e. 2.80 days post the GW T0), with the H filter starting on 2025-08-20T21:42:37 UT (i.e. 2.85 days post the GW T0)  and with the K filter starting on 2025-08-20T23:00:15 UT (i.e. 2.90 days post the GW T0). The exposure time for each filter was 50 minutes.

The transient host galaxy is clearly detected in all our images, while no source is detected at the position of AT2025ulz down to the following 3sigma upper limits (AB magnitudes, calibrated against the 2MASS catalogue and obtained after subtracting a 2D galaxy model obtained with galfit, Peng et al.,  2010, AJ, 139, 2097):

J > 21.7 mag
H > 21.3 mag
K > 20.5 mag

TITLE:   GCN CIRCULAR
NUMBER:  41492
SUBJECT: LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz
DATE:    25/08/22 11:30:51 GMT
FROM:    Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>

D. B. Malesani (DAWN/NBI and Radboud), A. Boye (DTU Space), L. Izzo (INAF/OAC and DARK/NBI), G. Leloudas (DTU Space), J. An (NAOC), X. Liu (NAOC), D. Xu (NAOC), M. Fraser (UCD), S. J. Brennan (MPE), J. Broe Bendsten (SDU), I. A. Koch (SDU), S. Lund Wagner (SDU), J. Magaard Knudsen (SDU), J. Hein Pedersen (SDU), J. P. U. Fynbo (DAWN/NBI), R. Holmberg Rasmussen (NOT and Aarhus), K. Valeckas (NOT and NBI), M. De Pasquale (U. Messina) report on behalf of a larger collaboration:

We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were started on 2025 Aug 20.875 UT (2.82 days after the GW trigger time). We adopted the filters r, i, and z (exposure times of 1200, 1200, and 1500 s, respectively).

Image subtraction using the Legacy sky survey as reference reveals a transient at coordinates (J2000):

RA = 15:51:54.19
Dec = +30:54:08.3

with an estimated error of < 0.5". These are consistent with the ZTF position (Stein et al., GCN 41414). The transient is located 1" southwest of the nucleus of its host galaxy, at a projected distance of 1.6 kpc (z = 0.0848; Karambelkar et al., GCN 41436; Banerjee et al., GCN 41476). Using nearby calibrators from the Legacy Survey, we obtained a provisional magnitude measurement of r = 23.2 ± 0.2 (AB), at a mean time of 2025 August 20.883 UT. We caution that we expect to refine our results after securing a deep template with the same instrument and filter.

TITLE:   GCN CIRCULAR
NUMBER:  41493
SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts  
DATE:    25/08/22 14:09:35 GMT
FROM:    James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>

S. J. Smartt, J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard).

We surveyed the skymap of the gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). Following on from the reported optical candidate AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480, D’Avanzo et al. GCN 41489), we have been taking targeted grizy imaging of this transient (Gillanders et al., GCN 41454). At this specific region we have proprietary data to make significantly deeper reference stacks than those that are available through the STScI public website, and monitoring with Pan-STARRS is ongoing.

For the rest of the northern portion of the GW skymap, we surveyed the region in the i-band over three nights: 2025 Aug 20, 21, 22. We covered ~288 square degrees and ~33% of the bilby.fits skymap 90% area each night, with multiple 45s (Aug 20) and 120s (Aug 21, 22) exposures. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Transient candidates were identified and run through our standard filtering procedures, including rejection of artefacts with machine learning tools and cross-matching with galaxy, stellar and solar-system catalogs (e.g., Smith et al., 2020, PASP, 132, 1;  Smartt et al., 2024, MNRAS 528, 2299).

The depth of our images are typically i ~ 21 - 21.5 per exposure, with the first night having better seeing and transparency conditions. We detected 68 candidate extragalactic transients, of which 30 were already registered on the IAU Transient Name Server (TNS) as having been discovered and detected before S250818k merger time. We have registered the other 38 sources on the TNS. To down-select for optical counterpart candidates of S250818k, we applied the following filters:

    1. Spatially associated with a host galaxy that has a photometric or spectroscopic redshift within the 2-sigma limit from LVK of 0.025 < z < 0.08,
    2. Has some indication of lightcurve evolution over the three nights of observation,
    3. We rejected all faint transients spatially coincident with the nucleus of galaxies as being due to either AGN variability or possible subtraction artefacts.

We found no plausible candidate for the optical counterpart of S250818k other than AT2025ulz. Six sources with plausible kilonova-like absolute magnitudes and in hosts with consistent redshifts (within errors) with S250818k were found (AT2025uuw, AT2025uso, AT2025uuf, AT2025utr, AT205usy, AT2025uxs), but their lightcurves are either rising or are flat and show no evolution over 48hrs, and they do not appear to be viable candidates at this point. Discovery details and magnitudes of the ATs can be found on the TNS.

Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys.

TITLE:   GCN CIRCULAR
NUMBER:  41500
SUBJECT: LIGO/Virgo/KAGRA S250818k: 3 GHz MeerKAT observations of AT2025ulz
DATE:    25/08/22 16:13:06 GMT
FROM:    Aishwarya L Thakur at INAF-IAPS, Rome <aishth@outlook.com>

G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report:

We observed the field of AT2025ulz (announced by Stein et al., GCN 41414), which is proposed as a candidate counterpart for the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440) with the MeerKAT radio telescope under the program SCI-20241101-GB-01 (PI: Bruni) at 3 GHz (S4 band) starting on Aug 21, 16:30 UT (at 3.6 days post-trigger) for a total of 2 hours (1.5 hours on source). J1939-6342 was used for flux scale calibration, and J1609+2641 for phase referencing. Data was processed with the SARAO Science Data Processor (SDP) continuum pipeline. The image RMS was 6.4 uJy/beam.

We detect radio emission at the position of the transient, and we measure an integrated flux density of 77 +/- 16 uJy (corresponding to a ~ 10-sigma detection). However, given the northern declination of the field, the angular resolution is insufficient to disentangle any potential contamination from the host with this epoch of observation alone.

Further MeerKAT observations are planned.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.
—------------------------
[1] INAF-IAPS

TITLE:   GCN CIRCULAR
NUMBER:  41501
SUBJECT: LIGO/Virgo/Kagra S250818k: STEP/T80N upper limits on 2025ulz
DATE:    25/08/22 17:12:58 GMT
FROM:    André Santos at Centro Brasileiro de Pesquisas Físicas (CBPF) <andsouzasanttos@gmail.com>

A. Santos (CBPF), C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), L. Santana-Silva (CBPF), P. Darc (CBPF), Gabriel Teixeira (CBPF), C. Mendes de Oliveira (IAG-USP) report on behalf of the STEP collaboration

We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; P. D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493), possibly associated with the GW superevent S250818k (LVK Collaboration, GCN 41437, GCN 41440), using the T80N 0.8-m robotic telescope using the 1.4 x 1.4 FoV T80N-Cam imager. 

The observations started on Aug 21, 21:46 UT (~3.85 days after trigger). We obtained 600s (2x300s)and 1200s (4x300s) stacked exposures in r and i bands respectively, with the T80-Cam centered at the position of the reported transient. Subtracting Pan-SSTARRS templates available at MAST archive using photpipe (Rest et al. 2005), we do not detect any sources in our difference images and derive 3-sigma limiting magnitudes of r > 22.9 and i > 22.8 mag for the transient.

We thank the OAJ Data Processing and Archiving Department (DPAD) for reducing and calibrating the OAJ data used in this work, as well as the distribution of the data products through a dedicated web portal.

TITLE:   GCN CIRCULAR
NUMBER:  41502
SUBJECT: LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Optical Detection of AT2025ulz
DATE:    25/08/22 17:59:55 GMT
FROM:    Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>

Rosa L. Becerra (U Rome), Alan M. Watson (UNAM), and Eleonora Troja (U Rome) report on behalf of a larger team:

We observed the field of AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440), with the OSIRIS instrument mounted on the GTC telescope. Our observations began at 2025-08-21 21:40 UTC (3.9 days after the trigger) and were carried out in the r and z filters with an average airmass of ~1.3 and excellent seeing.

After image subtraction against Legacy Survey templates (Dey et al. 2019), we detect the source with a preliminary magnitude of r~23.4 AB, consistent with the near-contemporaneous observations of Malesani et al. (GCN 41492). 
We also confirm a very red color, in agreement with earlier observations (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480).

Further observations are planned.

We thank the staff at the GTC, especially Antonio García Rodríguez, Antonio Marante Barreto, and Antonio Cabrera for the rapid execution of these observations. 

TITLE:   GCN CIRCULAR
NUMBER:  41503
SUBJECT: LIGO/Virgo/KAGRA S250818k: JinShan optical observations of AT2025ulz
DATE:    25/08/22 18:13:18 GMT
FROM:    J. An <jiean0813@foxmail.com>

J. An, X. Liu, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration:

We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Our first epoch observation started on 2025 Aug 20.61 UT, i.e., 2.55 days after the GW trigger time, in the Sloan g-, r-, and i- filters with 1 hr exposure for each filter. 

By image subtraction with the Legacy Survey template, we obtained a provisional magnitude of r = 22.8 +/- 0.3 (AB) at 2025 August 20.63 UT for AT2025ulz.

We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41504
SUBJECT: LIGO/Virgo/KAGRA S250818k: PRIME nIR observations of AT2025ulz
DATE:    25/08/22 23:39:31 GMT
FROM:    N. Passaleva at Sapienza University of Rome <niccolo.passaleva@uniroma1.it>

N. Passaleva  (U Rome),  J. Durbak (UMD), O. Guiffreda (UMD), E. Troja (U Rome), R.Hamada (Osaka U), A. S. Kutyrev (NASA/GSFC), D. Suzuki (Osaka U), T. Sumi (Osaka U), A. Idei (Osaka U), D. Buckley (SAAO), S. B. Cenko (NASA/GSFC)

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O'Connor et al. GCN 41452) possibly associated to the GW candidate S250818k (LVKC GCN 41437) in H filter and in J filter with PRIME ~87.62 hours and ~111.62 hours after the GW trigger.

Using  nearby stars from the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006) for calibration we estimate a 3-sigma limiting magnitude of H>20 AB for the image.
Within the transient's host galaxy we see no obvious point source, although template subtraction is required to place a meaningful upper limit to its brightness.

PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).

We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41505
SUBJECT: LIGO/Virgo/KAGRA S250818k: J-GEM follow-up observation for AT2025ulz
DATE:    25/08/23 05:37:03 GMT
FROM:    Mahito Sasada at Tokyo Institute of Technology <sasada@hp.phys.titech.ac.jp>

K. Taguchi (Kyoto U.), Y. Utsumi (NAOJ), M. Sasada (Science Tokyo), R. Itoh (Bisei Astronomical Observatory), T. Morokuma (Chiba Tech), K. Ohta (Kyoto U.), I. Takahashi (Science Tokyo), M. Tanaka (Tohoku), N. Tominaga (NAOJ), M. Yoshida (NAOJ), on behalf of J-GEM collaboration

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al., GCN 41414), which has been reported by Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502; An et al., GCN 41503, as a possible source to the Gravitational Wave Event S250818k (LVK collaboration GCN 41437; 41440), using TriCCS, a three-color simultaneous CMOS imager, on the 3.8m Seimei Telescope in Okayama, Japan.

We began the observation at 10:47:59 on Aug 20, 2025 (UT), 2.39 days after the merger.  Images in g, r, i, and z were integrated for 3899, 3899, 1919, and 1979 secs, respectively, resulting in coadded images with limiting magnitudes of 22.12, 21.87, 21.85, and 21.11 (AB, 5 sigma)

Differential image analysis against the PanSTARRS archival data (Chambers et al. 2016) using ZOGY (Zackay et al. 2016) has been attempted, but no obvious source has been revealed. 

A Sersic model of n=0.5 subtracted the host galaxy reasonably well, which revealed a possible remnant in g, r, i at the source location reported in GCN 41414; however, the forced aperture photometry at the reported location was consistent with the background sky fluctuation. 

Further analysis is underway.

TITLE:   GCN CIRCULAR
NUMBER:  41506
SUBJECT: LIGO/Virgo/KAGRA S250818k: HST nIR detection of AT 2025ulz
DATE:    25/08/23 14:34:22 GMT
FROM:    Eleonora <nora.gsfc@gmail.com>

Eleonora Troja (U Rome), Brendan O’Connor (Carnegie Mellon U.), and Rosa L. Becerra (U Rome) report on behalf of a larger team:

We observed the field of AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440) with the Hubble Space Telescope (PI: Troja) starting from 2025-08-22 20:04 UTC (~T_0+4.8 days).

The observations were carried out with the Wide-Field Camera 3 (WFC3) using the UVIS channel with the F336W filter and the IR channel with the F110W and F160W filters. Due to orbital constraints, we could acquire only short exposures of  ~120 s per filter.

At the location of AT2025ulz we clearly detect a source with preliminary AB magnitudes of:

F110W = 22.91 +/- 0.12
F160W = 22.8 +/- 0.3 (partial result based on 60 s exposure)
F336W = 23.9 +/ -0.4

The above values were derived from aperture photometry after subtracting a 2D GALFIT (Peng et al. 2004) model for the host galaxy, and are not corrected for Galactic extinction. 

The source is resolved in our nIR images and offset from the galactic center by ~0.9 arcsec, corresponding to a projected physical offset of ~1.5 kpc.  The measured nIR brightness matches well the kilonova AT2017gfo placed at the same distance of 388 Mpc. However, the blue color implied by the F336W observations is not consistent with a kilonova spectrum. 

At present we cannot determine whether the F336W flux is mostly dominated by the transient’s light, e.g. a rising supernova, or by host emission, e.g. underlying star-formation or a young star cluster. While the bulk of the galaxy is not detected in F336W, a star forming clump is still possible, and a clump is seen in the IR filters at the southern edge of the disk. Continued monitoring of the source is encouraged to assess its variability. 

We thank the HST staff, in particular Julia Roman-Duval, Patricia Royle, and William Januszweski for the rapid execution of these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41507
SUBJECT: LIGO/Virgo/KAGRA S250818k: Rebrightening detected with Gemini/GMOS
DATE:    25/08/23 15:18:41 GMT
FROM:    James Freeburn at University of North Carolina at Chapel Hill <jamesfreeburn54@gmail.com>

J. Freeburn (UNC), B. O’Connor (CMU), X. J. Hall (CMU), M. Busmann (LMU), I. Andreoni (UNC), A. Palmese (CMU), D. Gruen (LMU), L. Hu (CMU), T. Cabrera (CMU), K. Kunnumkai (CMU), A. Amsellem (CMU) report on behalf of a larger collaboration: 

Using Gemini/GMOS-N, starting at 2025-08-23 05:30:48 UTC, we observed the source AT 2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414) discovered during ZTF follow-up of the low-significance gravitational wave event S250818k (GCN 41437, 41440; https://gracedb.ligo.org/superevents/S250818k), which is a candidate binary neutron star merger. We took images in g and i bands.

We subtracted Gemini/GMOS-N observations taken on the previous night (starting at 2025-08-22 05:43:02) from these observations.  In the g-band, no source is detected in the difference image.   The i-band difference image reveals a positive source at the location of AT2025ulz/ZTF25abjmnps.  This positive detection corresponds to an increase in brightness of 0.63+/-0.09 microJanksy, which indicates rising emission in i-band at this phase.

Further observations are planned and additional follow-up is encouraged to further monitor the transient’s evolution.

We thank the staff of the Gemini Observatory, in particular Jen Andrews, Brian Lemaux, and Jen Miller, for their excellent support in rapidly scheduling these observations. 

TITLE:   GCN CIRCULAR
NUMBER:  41518
SUBJECT: LIGO/Virgo/KAGRA S250818k: COLIBRÍ confirmation of rebrightening of AT2025ulz
DATE:    25/08/24 06:21:08 GMT
FROM:    Alan Watson at UNAM <alan@astro.unam.mx>

Camila Angulo (UNAM), Alan M. Watson (UNAM), Damien Dornic (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Fredd Sánchez Álvarez (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of the AT2025ulz (Stein et al., GCN Circ. 41414) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed at two epochs, from 2025-08-22 03:19 to 05:29 UTC (4.13 days after the trigger) and from 2025-08-24 03:29 to 05:41 UTC (6.14 days after the trigger). At both epochs we obtained 96 minutes of exposure in the i filter.

The data were reduced and coadded with custom software and then analyzed in STDWeb (Karpov 2021). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

After subtracting a Pan-STARRS DR2 template image from our first epoch image (4.13 days after the trigger), we detect a source at the reported position of AT2025ulz with

i = 23.1 +/- 0.25

After subtracting a Pan-STARRS DR2 template image from our second epoch image (6.14 days after the trigger), we detect a source at the reported position of AT2025ulz with

i = 22.0 +/- 0.1

Subtracting the first epoch image from the second epoch image, we find a brightening that corresponds to

Delta i = 22.7 +/- 0.2

We caution that these analyses are preliminary, especially the photometry after subtracting the shallower PS template images. Nevertheless, our results would appear to confirm the rebrightening reported by Freeburn et al. (GCN Circ. 41507).

Further observations are planned.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

TITLE:   GCN CIRCULAR
NUMBER:  41519
SUBJECT: LIGO/Virgo/KAGRA S250818k: CFH/MegaCam follow-up observation for AT2025ulz
DATE:    25/08/24 07:14:08 GMT
FROM:    Sarah Antier at OCA <sarah.antier@oca.eu>

S. Antier (OCA/IJCLAB), M. Pillas (Uliege), D. Akl (NYUAD), S. Karpov (FZU), M. Coughlin (UMN), P. Hello (IJCLAB), A. Jacquesson (IJCLAB) report on behalf on ZTF collaboration:

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al., GCN 41414) over several nights, which has been reported by Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502; An et al., GCN 41503, Angulo et al., GCN 41518 as a possible source to the Gravitational Wave Event S250818k (LVK collaboration GCN 41437; 41440). The observations were carried out using MegaCam on the CFHT.

We report here our preliminary measurement in r-band taken around 2025-08-21 06:50 UTC by co-adding several images and analyzed with STDPipe (Karpov 2025) with photometric calibration against Pan-STARRS. The quality of the sky during that night was not ideal.

r = 22.85 +/- 0.15 (AB)

Differential image analysis against archival CFH data has been performed. 

Further observations are planned.

We would like to thank the CFHT staff enabling these timely observations, in particular Nadine Manset and Heather Flewelling.

TITLE:   GCN CIRCULAR
NUMBER:  41528
SUBJECT: LIGO/Virgo/KAGRA S250818k: Swift Observations of AT 2025ulz - Second Epoch
DATE:    25/08/24 16:00:28 GMT
FROM:    Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>

Rosa L. Becerra (U Rome), Eleonora Troja (U Rome) and Simone Dichiara (PSU) report:

We requested a second epoch of ToO observations of AT2025ulz/ZTF25abjmnps (Stein et al.; GCN 41414) discovered within the localization of the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440).

The observations started on 2025-08-22 at 21:56 UT, approximately 4.9 days after the trigger, with a total exposure of 3.2 ks. At the transient position, we detect no X-ray source down to a 3-sigma upper limit of 0.004 cts/s (0.3-10 keV), consistent with the result reported by Hall et al. (GCN 41453). Using a power-law with nH=2.2E+20 and a photon index=2, we convert the count rate into an unabsorbed flux of 1.5e-13 erg/cm2/s, which corresponds to a luminosity of 3e42 erg/s at z=0.0848 (Karambelkar et al. GCN 41436). 

In simultaneous UVOT observations performed with the u filter, we identify some faint diffuse emission likely related to the host galaxy and place 3-sigma upper limit u > 21 AB at the transient’s position, in agreement with the faint detection reported by Troja et al. (GCN 41506).

We thank the Swift team and the PI for promptly scheduling and making these observations possible.

TITLE:   GCN CIRCULAR
NUMBER:  41532
SUBJECT: LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of SN 2025ulz as a type II supernova
DATE:    25/08/24 20:46:51 GMT
FROM:    Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>

Smaranika Banerjee (Stockholm University), Maria-Teresa Botticella (INAF - Capodimonte Obs.), Seán J. Brennan (MPE), Enrico Cappellaro (Padua Obs.), Ting-Wan Chen (NCU Taiwan), Paolo D'Avanzo (INAF - Brera Obs.), Valerio D’Elia (ASI-SSDC), Massimiliano De Pasquale (Univ. Messina), Rob A. J. Eyles-Ferris (Leicester), Morgan Fraser (UCD), James H. Gillanders (Oxford), Ben Gompertz (Birmingham), Nusrin Habeeb (Leicester), Luca Izzo (INAF/OAC and DARK/NBI), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud and Warwick), Daniele Bjørn Malesani  (DAWN/NBI and Radboud), Antonio Martin-Carrillo (UCD), Matt Nicholl (QUB), Sam Oates (Lancaster U.), Silvia Piranomonte (INAF-OAR), Luigi Piro (INAF-IAPS), Andrea Rossi (INAF - Bologna), Om Sharan Salafia (INAF - Brera Obs.), Nikhil Sarin (Cambridge), Steve Schulze (Northwestern), Avinash Singh (Stockholm University), Stephen J. Smartt (Oxford), Albert Sneppen (DAWN/NBI), Jesper Sollerman (Stockholm), Danny Steeghs (Warwick), Nial R. Tanvir (Leicester), Aishwarya L. Thakur (INAF-IAPS), report for the ENGRAVE collaboration:

We report preliminary analysis from follow-up ESO VLT observations obtained by the ENGRAVE collaboration of SN 2025ulz (Stein et al., GCN 41414), which was discovered within the 3D localization region of the GW alert S250818k (LVK Collaboration, GCNs 41437, 41440).

Starting on 2025-08-24 00:03:40 UT, we observed the field of SN 2025ulz using the ESO VLT UT4 (Yepun) equipped with the MUSE integral-field spectrograph. A series of 4x700 s exposures were taken for this observation.

From a preliminary reduction, extraction of a spectrum at the position of SN 2025ulz reveals a continuum well detected over the full wavelength range of 5000 to 9000 Å. The spectrum shows a prominent broad feature which we interpret as a P-Cygni profile of the H-alpha line, similar to that observed in young type II or IIb supernovae. The peak of the emission and the absorption trough is consistent with the host galaxy redshift of z = 0.0848 (ENGRAVE GCN 41476,  Karambelkar et al., GCN 41436) and an expansion velocity of around 12,000 km/s. There is weaker evidence for an H-beta P-Cygni profile. Plausible matches to young type II and IIb SNe are found with SNID-SAGE (Stoppa et al. in prep, a new enhanced, python version of SNID), although the transient object continuum is contaminated by the host and the absorption line strengths are therefore diluted. We suggest that SN 2025ulz is a type II supernova (of unconfirmed subtype at this point) and is therefore unrelated to S250818k.

We acknowledge the excellent support from the ESO staff in Paranal, in particular Camila de Sa Freitas, Israel Blanchard and Sam Kim.

TITLE:   GCN CIRCULAR
NUMBER:  41534
SUBJECT: LIGO/Virgo/KAGRA S250818k:  MASTER  predicovery limits of the AT2025ulz/ZTF25abjmnps KN candidate
DATE:    25/08/25 08:21:58 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>



V.Lipunov, E.Gorbovskoy, A.Kuznetsov, N.Tiurina, D.Vlasenko, P.Balanutsa,
I.Panchenko, A.Sankovich, A.Chasovnikov K.Zhirkov, G.Antipov, I.Gorbunov, A.Sosnovskij, Yu.Tselik, Ya.Kechin, V.Senik, V.Topolev (Lomonosov MSU),
O.Gress, N.Budnev, O.Ershova (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix
Aguilar (OAFA),
D.Buckley (SAAO),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)

Global MASTER  robotic net [1] observed the field with AT2025ulz/ZTF25abjmnps,  reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440) before and after trigger time.

 At the location of AT2025ulz the twin MASTER600-Tunka telescope (Cmos QHY 6060, FOW = 18 square degress)   did not  detect a source with next 5-sigma limits:

 Date    UT   Tstart-Ttrig  Exp    Filter  tube    lim
       (middle)   (day)     (s)

250815 14:55:18  -3.12       60     Clear   East    19.8
250822 16:52:05   3.95     8x60     Clear   West    20.9
250823 15:37:06   4.90    38x60     Clear   West    21.8

The first observation was a lucky coincidence.
Since there was no automatic LVC alert,
subsequent observations had to be carried out in the Internet telescope mode, taking into account the specific quality of the weather.

[1] V.M. Lipunov, V.G. Kornilov, E.Gorbovskoy, N. Tiurina & A.Kuznetsov,
Astronomical Robotic Networks and Operative Multichanel Astrophysics,
Lomonosov MSU PRESS, 591pp.
http://www.pereplet.ru/lipunov/625.html#625

TITLE:   GCN CIRCULAR
NUMBER:  41535
SUBJECT: LIGO/Virgo/KAGRA S250818k: FTW Observations Show Continued Reddening of AT 2025ulz
DATE:    25/08/25 09:09:19 GMT
FROM:    Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>

Malte Busmann (LMU), Xander J. Hall (Carnegie Mellon U.), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), and Mansi Kasliwal (Caltech) report:

We observed the source AT 2025ulz reported by Stein et al. (GCN 41414)  possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, i, and J bands simultaneously starting at 2025-08-24T19:33:25 for 20 x 180 s with a few interruptions due to clouds. The difference imaging with templates from the Legacy Survey for the g-band and PS1 for the i-band, we report that AT 2025ulz has g - i ~ 1.7 AB mag.

We note our differenced i-band brightness is consistent with observations reported by Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518).

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

Further analysis is underway.

We thank Christoph Ries from the Wendelstein Observatory staff for obtaining these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41538
SUBJECT: LIGO/Virgo/KAGRA S250818k: Continued Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
DATE:    25/08/25 10:04:54 GMT
FROM:    Mansi Kasliwal at Caltech <mansikasliwal@gmail.com>

Mansi M. Kasliwal (Caltech), Viraj Karambelkar (Caltech), Christoffer Fremling (Caltech), Tomas Ahumada (Caltech), Xander J. Hall (CMU), Daniel A. Perley (LJMU), Shreya Anand (Stanford), Chang Liu (Northwestern), Kaustav Das (Caltech), Varun Bhalerao (IIT-B), Vishwajeet Swain (IIT-B), Aditya Saikia (IIT-B)

report on behalf of a larger ZTF and GROWTH collaboration

We continue to observe ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) with the LRIS spectrograph on Keck I. Our latest epoch starts at UTC 2025-08-25 06:50. We reduce the data with lpipe (Perley et al. 2019) and subtract a spectrum at an earlier epoch to understand the evolution (Karambelkar et al. GCN 41436).

As reported by Banerjee et al. GCN 41532, we also see a feature with an absorption minimum at 6720A. If this is Halpha, this would imply a velocity of 17000 km/s at z=0.0848 (we measure a higher velocity than what was reported in Banerjee et al. GCN 41532). 

However, we posit the classification of this transient as a supernova or a kilonova currently remains ambiguous. If it is a supernova, it is an unusual supernova for at least four reasons: (i) There is no robust (rlap>5) spectral match in the SNID library, (ii) No supernova light curve in the ZTF supernova library matches especially the blue-to-red color evolution at this early phase (e.g. Busmann et al. GCN 41535), (iii) The feature at 6720A is not smooth in shape and has additional structure, (iv) There appear to be other weak, broad undulations in the spectra. If it is a kilonova, all photometric data in-hand continue to be consistent with this hypothesis, however, additional theoretical modeling may be needed to fully explain the spectra.    

Looking ahead, we encourage continued follow-up of the panchromatic light curve, including the radio and X-ray bands, to unambiguously classify this source.    

TITLE:   GCN CIRCULAR
NUMBER:  41540
SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS imaging confirms re-brightening of SN2025ulz
DATE:    25/08/25 13:07:11 GMT
FROM:    James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>

J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard).

We have been observing the optical transient SN2025ulz (Banerjee et al., GCN 41532), formerly AT2025ulz (Stein et al., GCN 41414), the candidate optical counterpart associated with the sub-threshold gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560); see also Gillanders et al. (GCN 41454) and Smartt et al. (GCN 41493).

Our observations span grizy-bands, but here we report on the optical re-brightening evident in our i-band observations. On each of MJDs 60909.26, 60910.26, 60911.26 and 60912.26, we observed SN2025ulz for a total exposure time of 1800s. The images were processed with the Pan-STARRS pipeline, where, after astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). For this process, we utilise proprietary data to compile significantly deeper reference stacks (~3400s) than those that are publicly available through the STScI website (~300s) and our stacked target images (1800s).

From these difference images, we measure the following preliminary AB magnitudes:

MJD         t-t0 (days)    Filter    AB mag
60909.26    4.2            i         22.4 +/- 0.3
60910.26    5.2            i         22.5 +/- 0.3
60911.26    6.2            i         22.1 +/- 0.2
60912.26    7.2            i         21.6 +/- 0.1

Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437).

SN2025ulz has risen in our i-band observations by ~0.5 mag (~0.9 mag) in the last one day (two days). While there are barriers to accurately measuring the photometry of this source (e.g., it is relatively faint; it lies close to the bright nucleus and body of the host), our photometry values, and the inferred re-brightening, are robust, given our access to deep reference stacks. We note that these are vital to accurately infer the true brightness of the transient, as performing difference imaging with a shallower reference image can lead to systematic errors (as much as ~0.7 AB mag, in the specific case of SN2025ulz utilising the public reference stack available through the STScI website).

Re-brightening of SN2025ulz has been previously noted by Liu et al. (GCN 41461), Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518).

Our observed re-brightening supports the interpretation of SN2025ulz being a young SN, based on the VLT spectral observations reported by ENGRAVE (Banerjee et al., GCN 41532). Our consistent i-band rise is likely to suggest it is now on a radioactively powered rise following an initial shock-cooling phase, and thus indicates that SN2025ulz is likely not the optical counterpart to S250818k. 

Further Pan-STARRS observations are continuing to determine if the colour and lightcurve shape remains consistent with a type II or IIb supernova, to compare with the conclusions of Kasliwal et al. (GCN 41538).

Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys.

TITLE:   GCN CIRCULAR
NUMBER:  41541
SUBJECT: LIGO/Virgo/KAGRA S250818k:  nature of the AT2025ulz/ZTF25abjmnnps OT
DATE:    25/08/25 13:17:47 GMT
FROM:    Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>


V.M. Lipunov, (Lomonosov MSU),


In several recent telegrams (GCN 41532, 41538) the nature of the AT2025ulz/ZTF25abjmnps source is discussed. An alternative between the KN and SN II models has emerged. The unusual observable properties of the optical flare are emphasized. This alternative seems strange. For example, where can SN II appear in an elliptical galaxy?

In this regard, I propose  to include a third possible model, namely the merger of two heavy white dwarfs (O-Ne-Mg dwarfs) turning into a neutron star - a dumbbell with powerful radiation of gravitational waves [1].

As was shown earlier, such a phenomenon can generate an unusual form of gravitational wave impulse, which can be taken for a phenomenon caused by a terrestrial artifact. Is this not related to the fact that the initial probability of a terrestrial nature turned out to be more than 70%?


[1] Lipunov V.M., Double O-Ne-Mg white dwarfs merging as the source of the powerful gravitational waves for LIGO/VIRGO type interferometers., New Astronomy, Volume 56, p. 84-85, 2017 https://doi.org/10.1016/j.newast.2017.04.013

TITLE:   GCN CIRCULAR
NUMBER:  41542
SUBJECT: LIGO/Virgo/KAGRA S250818k: VLA upper limits on AT2025ulz
DATE:    25/08/25 14:13:32 GMT
FROM:    muskan.yadav@students.uniroma2.eu

R. Ricci, M. Yadav, E. Troja (U Rome) report on behalf of the ERC BHianca team:

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) within the localization of the candidate gravitational wave event S250818k (LVKC GCN 41437; GCN 41440) with the Very Large Array in S-band and C-band at the centre frequency of 3 GHz and 6 GHz respectively with a bandwidth of 2 GHz and 4 GHz on Aug 24st 2025, 5.9 days after the GW trigger. 

A possible radio counterpart with flux 77 +/- 16 microJy/beam at 3 GHz was claimed by Bruni et al. (GCN 41500). 
After reducing the data using the latest version of the CASA VLA pipeline (v6.6.1) and standard imaging procedures, we found no detection at the transient position down to a 3-sigma flux density upper limit of about 40 microJy/beam and 25 microJy/beam for 3 GHz and 6 GHz respectively. 

We thank the VLA staff for promptly executing the observations.

TITLE:   GCN CIRCULAR
NUMBER:  41544
SUBJECT: LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Confirmation of Rebrightening of AT2025ulz
DATE:    25/08/25 19:06:17 GMT
FROM:    Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>

Rosa L. Becerra (U Rome), Yuhan Yang (U Rome), Alan M. Watson (UNAM), Eleonora Troja (U Rome) and William H. Lee (UNAM) report on behalf of a larger team:

We continued monitoring AT2025ulz/ZTF25abjmnps, initially reported by Stein et al. (GCN 41414) within the localization region of the candidate gravitational wave event S250818k (LVK Collaboration, GCN 41437, 41440), using the OSIRIS instrument mounted on the GTC telescope. Our observations commenced at 2025-08-24 21:18 UTC (6.8 days after the trigger) and were carried out in the g, r, i and z filters at an average airmass of ~1.3.

By comparison with our first epoch at T+3.9 days (Becerra et al., GCN 41502), we confirm a significant rebrightening in both the r and z bands, measuring: 

delta_r = 0.9 +/- 0.3
delta_z = 0.6 +/- 0.1

Our observations are in agreement with the rebrightening observed in i-band (Freeburn et al. GCN 41507; Angulo et al. GCN 41518; Busmann et al. GCN 41535; Gillanders  et al. GCN 41540), a behavior that is not expected for kilonova emission. 

Further observations are planned. 

We thank the staff at the GTC, especially Antonio García Rodríguez, Antonio Marante Barreto, and Antonio Cabrera for the rapid execution of these observations. 

TITLE:   GCN CIRCULAR
NUMBER:  41548
SUBJECT: LIGO/Virgo/KAGRA S250818k: SAO RAS BTA z-band observations of AT2025ulz
DATE:    25/08/26 11:49:10 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>

N. Pankov (HSE, IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), S. Kotov (SAO RAS), A. Volnova (IKI), P. Minaev (IKI) report on behalf of the IKI-GRB-FuN collaboration:

We observed the field of an optical transient AT2025ulz detected by ZTF (Stein et. al, GCN 41414) in the localization region of a candidate binary neutron star merger S250818k (LVK, GCN 41437). The observations with BTA 6-meter telescope of SAO RAS started on 2025-08-22 at 17:18:46 UT, i.e. ~4.67 days since LVK trigger. We obtained 2.4 ks exposure in the z'-band using the Scorpio-2 multi-mode focal reducer (Afanasiev & Moiseev,  2011, BaltA, 20, 363) installed in the prime focus of the telescope. We applied image subtraction using LS DR10 z-band image as reference. We clearly detect the optical source in the difference image at the position given by ZTF (Stein et. al, GCN 41414). The preliminary photometry of the source is presented below:

Date       UTstart  Exptime,s T-T0,days Filter OT    Err  UL(3sigma) FWHM,"
---------- -------- -------   -------   --     ----- ---- ----       ---
2025-08-22 17:18:46 2400.13   4.67963   z'     24.09 0.24 24.3       1.8

The photometry was calibrated using comparison stars from SDSS-DR12 and not corrected for the Galactic extinction. The OT was also followed by (Busmann et. al, GCN 41421; Hall et. al, GCN 41433; Karambelkar et. al, GCN 41436; Nicholl et. al, GCN 41439; Srivastav et. al, GCN 41451; O’Connor et. al, GCN 41452; Gillanders et. al, GCN 41454; Mo et. al, GCN 41456; Liu et. al, GCN 41461; Ackley et. al, GCN 41468; Klose et. al, GCN 41474; Banerjee et. al, GCN 41476; Perley et. al, GCN 41480; D'Avanzo et. al, GCN 41489; Malesani et. al, GCN 41492; Smartt et. al, GCN 41493; Santos et. al, GCN 41501; Becerra et. al, GCN 41502; An et. al, GCN 41503; Passaleva et. al, GCN 41504; Taguchi et. al, GCN 41505; Troja et. al, GCN 41506; Freeburn et. al, GCN 41507; Angulo et. al, GCN 41518; Antier et. al, GCN 41519; Banerjee et. al, GCN 41532; Lipunov et. al, GCN 41534; Busmann et. al, GCN 41535; Kasliwal et. al, GCN 41538; Gillanders et. al, GCN 41540; Becerra et. al, GCN 41544). We note that our measurements may be affected during image subtraction and should be considered as a lower limit on the source brightness.

TITLE:   GCN CIRCULAR
NUMBER:  41577
SUBJECT: LIGO/Virgo/KAGRA S250818k: uGMRT 1.3 GHz observations of AT2025ulz
DATE:    25/08/28 14:58:07 GMT
FROM:    pchandra@nrao.edu

G. Bruni (INAF-IAPS), 
P. Chandra (NRAO), 
S. Chatterjee(Rhodes University)
C. Christy (Arizona),
R. Kale (NCRA-TIFR), 
T. Laskar (Utah), 
S. Mohnani (IIT Indore),
S. R. Das (IIST), 
L. Resmi (IIST), 
Roberto Ricci (URome)
and
E. Troja (URome)
report on behalf of multiple teams:

"We observed AT2025ulz (Stein et al. GCN 41414) with the Giant Metrewave Radio Telescope (GMRT) under programs 48_081 (PI: Mohnani), 48_090 (PI: Troja), 48_096 (PI: Chandra), 48_097 (PI: Bruni), 48_120 (PI: Laskar), and 48_151 (PI: Resmi) beginning on 2025 Aug 26 at 13:30 UT (~8.5 days after the LVK trigger) for 2 hours at Band 5 (1.3 GHz).

In preliminary analysis, we do not detect significant radio emission at the position of AT2025ulz or the host. The image rms is ~ 23 microJy/beam, resulting in a 5-sigma upper limit of 115 uJy, at an angular resolution of 2.5x2.0 arcsec.

Together with the VLA 3 GHz non-detection at the position of AT2025ulz (<40 uJy, GCN 41542), our GMRT 1.3 GHz upper limit suggests that the radio emission previously detected with MeerKAT at lower angular resolution may include a contribution from the host galaxy (as already noted in the GCN 41500).

We thank the GMRT staff for scheduling and executing these observations. GMRT is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.

TITLE:   GCN CIRCULAR
NUMBER:  41594
SUBJECT: LIGO/Virgo/KAGRA S250818k: MeerKAT detection of an increase in radio flux from AT2025ulz
DATE:    25/08/29 18:07:17 GMT
FROM:    Gabriele Bruni at INAF <gabriele.bruni@inaf.it>

G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report:

We re-observed the field of AT2025ulz (announced by Stein et al., GCN 41414), which is proposed as a candidate counterpart for the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440) with the MeerKAT radio telescope under the program SCI-20241101-GB-01 (PI: Bruni) at 3 GHz (S4 band) starting on Aug 28, 14:20 UT (at 10 days post-trigger) for a total of 4 hours (3.23 hours on source). J1939-6342 was used for flux scale calibration, and J1609+2641 for phase referencing. Data was processed with the SARAO Science Data Processor (SDP) continuum pipeline. The image RMS was 5.3 uJy/beam.

We detect a clear flux increase in the radio emission with respect to the first epoch at 3.6 days post trigger (GCN 41500), with an integrated flux density of 157 +/- 9 uJy. The increment with respect to the first epoch is thus ~80 uJy.

Further MeerKAT observations are planned.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.
—------------------------
[1] INAF-IAPS

TITLE:   GCN CIRCULAR
NUMBER:  41666
SUBJECT: LIGO/VIRGO/KAGRA S250818k: No evidence for radio variability of AT2025ulz in MeerKAT 3 GHz data
DATE:    25/09/03 04:23:58 GMT
FROM:    Kunal Mooley at NRAO,Caltech <kmooley@caltech.edu>

Lauren Rhodes (McGill U.), Oleg Smirnov (Rhodes U. / SARAO), Kunal Mooley (IIT Kanpur, Caltech), Patrick Woudt (U. Cape Town)

We reprocessed both MeerKAT S-band (3 GHz) datasets of the AT2025ulz (Stein et al., GCN 41414) field reported previously (Bruni et al. GCNs 41594, 41500) in the context of the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440). Our reprocessing reveals that in both datasets, from Aug 28 (10 days post-GW-event) and Aug 21 (4 days post-GW-event), the source has a constant integrated flux density of ~90+/10 uJy. Thus, we do not find any evidence for radio variability. Further, the extended nature of the radio source indicates that the emission is host galaxy dominated.

We caution all users of the SARAO Science Data Processor (SDP) continuum pipeline that the pipeline results must be carefully assessed before dissemination across the astronomical community. In the Aug 28 SDP images, there were large-scale artefacts, likely RFI-induced, that may be responsible for the excess flux measurement reported in GCN 41594.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.

TITLE:   GCN CIRCULAR
NUMBER:  41364
SUBJECT: LIGO/Virgo/KAGRA S250814bg: Identification of a GW compact binary merger candidate possibly associated with GRB 250814A detected by Fermi GBM and Swift BAT
DATE:    25/08/14 10:31:50 GMT
FROM:    Sreeta Roy at University of Warsaw <sreeta.roy@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration with the Fermi GBM team and Swift team report:

We identified the compact binary merger candidate S250814bg during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-14 07:35:05.587 UTC (GPS time: 1439192123.587). The candidate was found by the GstLAL [1] analysis pipeline.

Based on the analysis of gravitational-wave data alone, this candidate does not meet our criteria for a high-significance public alert as its false alarm rate is estimated by the online analysis to be 0.00012 Hz or about one in 2 hours. However, a search performed by the RAVEN pipeline found a significant coincidence between this candidate and Fermi GBM trigger with ID 776849714, which is associated with GRB 250814A. The event’s properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250814bg

The classification of the GW signal, in order of descending probability, is Terrestrial (>99%), BBH (<1%), NSBH (<1%), or BNS (<1%).

Excess noise was present in the L1 detector at the time of this candidate, which may affect the parameters or the significance of the candidate.  

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 3%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two GW-only sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 2 minutes after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 6 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 7867 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 13504 +/- 6537 Mpc (a posteriori mean +/- standard deviation).

A search performed by the RAVEN pipeline [4] found a temporal coincidence between S250814bg and a Fermi GBM trigger with ID 776849714, which is associated with GRB 250814A (Fermi GBM Team, GCN Circular 41357). The GRB trigger time is 3.7 seconds after the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info before trials are applied is 7.5e-09 Hz, or about one in 4 years.

Combined sky maps are also available:
 * combined-ext.multiorder.fits,0, an initial localization, distributed via GCN and SCiMMA notices about 2 minutes after the candidate event time.
 * combined-ext.multiorder.fits,1, an initial localization, distributed via GCN and SCiMMA notices about 6 minutes after the candidate event time.
 * combined-ext.multiorder.fits,2, an initial localization, distributed via GCN and SCiMMA notices about an hour after the candidate event time.

For the combined-ext.multiorder.fits,2 sky map, the 90% credible region is 471 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence before trials are applied is 1.5e-08 Hz, or about one in 2 years. After including the trials factor of 30, the joint false alarm rate becomes 4.5e-07 Hz, or about 14 per year.

Moreover, the GW candidate is 6 seconds after the Swift-BAT trigger 1342439, associated with the same GRB 250814A (Swift Team, GCN Circular 41358). The Swift-BAT GRB is spatio-temporal coincident with the Fermi GBM one, therefore they can be reliably considered the same astrophysical event. Considering the Swift-BAT localization, the estimated joint false alarm rate between the Swift-BAT and the GW candidate for the spatial and temporal coincidence before trials are applied is 1.0e-08 Hz, or about one in 3 years.  After including the trials factor of 30, the joint false alarm rate becomes 3.0e-07 Hz, or about 9 per year.

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
 [4] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060

TITLE:   GCN CIRCULAR
NUMBER:  41389
SUBJECT: GRB 250814A: Mondy optical observations of the candidate counterpart to LVK S250814bg
DATE:    25/08/15 10:55:07 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>

N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN:

We observed the field of GRB 250814A detected by Fermi/GBM (The Fermi GBM team, GCN 41357), Swift/BAT (Caputo et. al, GCN 41358), which was temporally and spatially coincident with a GW event S250814bg detected by LVK (GCN 41364). The observations were carried out with the 1.5m AZT-33IK telescope of the Solar Sayan Observatory (Mondy). The series of 30x120 s images were captured in the R-filter starting on 2025-08-14 at 17:49 UT, i.e. ~0.44 days after the GBM trigger. In the co-add image of 28x120 s we do not detect any optical source at the position of the IR candidate (Karambelkar et. al, GCN 41377; Castro-Tirado et. al, GCN 41381; Ahumada et. al, GCN 41382). The preliminary upper limit on a candidate optical counterpart in the co-add image is presented below:

Date       UT start     t-T0        Exp.  Filter OT   Err     UL
                     (mid, days)  (n x s)                  (3sigma)
2025-08-14 17:51:08    0.44721     28x120   R    n/d  n/d    22.7

The photometry was calibrated using nearby stars from the PS1 DR2 catalog (R mags were obtained via Lupton 2005 transformations) and not corrected for the Galactic extinction. The candidate counterpart position was also observed in the optical and IR bands by (Becerra et. al, GCN 41362; de Ugarte Postigo et. al, GCN 41367; Karambelkar et.al, GCN 41372; Salazar Manzano et. al, GCN 41375; Jiang et. al, GCN 41387), which resulted in upper limits.

TITLE:   GCN CIRCULAR
NUMBER:  41336
SUBJECT: LIGO/Virgo/KAGRA S250813k: Identification of a GW compact binary merger candidate
DATE:    25/08/13 02:36:38 GMT
FROM:    neil.lu@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250813k during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-13 02:12:00.444 UTC (GPS time: 1439086338.444). The candidate was found by the cWB [1], GstLAL [2], MBTA [3], MLy [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S250813k is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.3e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250813k

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 21 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(09h01m, +73d31m, 5.03d, 1.32d, 107.20d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 932 +/- 209 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [4] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.0240

TITLE:   GCN CIRCULAR
NUMBER:  41350
SUBJECT: LIGO/Virgo/KAGRA S250813k: Updated Sky localization
DATE:    25/08/13 21:55:57 GMT
FROM:    Sylvia Biscoveanu at Northwestern CIERA <sylvia.biscoveanu@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250813k (GCN Circular 41336). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250813k

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 13 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(09h02m, +74d05m, 3.72d, 1.09d, 106.41d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 708 +/- 159 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41351
SUBJECT: LIGO/Virgo/KAGRA S250813k: NED Galaxies in the 4-Update Localization Volume
DATE:    25/08/13 22:45:01 GMT
FROM:    David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>

David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)

On behalf of the NASA/IPAC Extragalactic Database (NED) Team.

We spatially cross-matched the LVK S250813k-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 155 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:

Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250813k/4                       
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250813k/4/20

The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.

|                  objname|            ra|           dec|objtype|    DistMpc|DistMpc_unc|       m_NUV|   m_NUV_unc|        m_Ks|    m_Ks_unc|        m_W1|    m_W1_unc|    P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|
|WISEA J091600.60+754837.0|     139.00362|      75.80978|      G|     743.12|       null|        null|        null|      12.790|       0.114|      11.225|       0.006|1.02e-06|  6.45e-08|
|WISEA J085943.95+740702.1|     134.93483|      74.11685|      G|     744.63|       null|        null|        null|      13.083|       0.113|      12.827|       0.010|3.05e-06|  4.44e-08|
|WISEA J090329.43+731952.8|     135.87263|      73.33136|      G|     769.02|       null|        null|        null|      13.434|       0.144|      12.868|       0.010|2.18e-06|  3.25e-08|
|WISEA J090156.31+745448.6|     135.48465|      74.91352|      G|     608.62|       null|        null|        null|      13.563|       0.165|      13.395|       0.025|5.01e-06|  2.88e-08|
|WISEA J090057.52+734556.4|     135.23969|      73.76568|      G|     531.03|       null|        null|        null|      12.743|       0.128|      12.700|       0.011|3.30e-06|  2.75e-08|
|WISEA J085751.67+730314.9|     134.46530|      73.05415|      G|     560.91|       null|      20.368|       0.159|      13.715|       0.146|      12.494|       0.013|2.36e-06|  2.65e-08|
|WISEA J091306.11+753712.2|     138.27548|      75.62007|      G|     501.03|       null|        null|        null|      12.896|       0.123|      12.393|       0.009|2.67e-06|  2.62e-08|
|WISEA J085557.92+730643.9|     133.99135|      73.11222|      G|     729.14|       null|        null|        null|      13.591|       0.164|      13.317|       0.012|2.84e-06|  2.53e-08|
|WISEA J090309.87+740351.7|     135.79113|      74.06438|      G|     690.65|       null|        null|        null|      13.836|       0.169|      13.566|       0.025|4.00e-06|  2.53e-08|
|WISEA J090722.74+745314.9|     136.84477|      74.88750|      G|     560.31|       null|        null|        null|      13.366|       0.150|      13.349|       0.025|4.85e-06|  2.47e-08|
|WISEA J090920.49+732006.8|     137.33539|      73.33525|      G|     649.99|       null|        null|        null|      13.738|       0.164|      12.081|       0.010|1.11e-06|  2.44e-08|
|WISEA J085409.21+730852.6|     133.53838|      73.14795|      G|     890.70|       null|        null|        null|      13.089|       0.117|      12.653|       0.012|9.25e-07|  2.28e-08|
|WISEA J090616.44+751235.5|     136.56851|      75.20987|      G|     653.18|       null|        null|        null|      13.689|       0.176|      13.670|       0.028|4.42e-06|  2.27e-08|
|WISEA J090229.38+732949.1|     135.62244|      73.49698|      G|     623.66|       null|      20.646|       0.194|      13.692|       0.161|      13.281|       0.013|3.29e-06|  2.21e-08|
|WISEA J090910.90+751628.1|     137.29543|      75.27447|      G|     637.20|       null|        null|        null|      13.552|       0.144|      13.679|       0.022|4.28e-06|  2.08e-08|
|WISEA J090759.32+734442.1|     136.99718|      73.74504|      G|     667.18|       null|        null|        null|      13.483|       0.149|      13.309|       0.013|2.68e-06|  2.01e-08|
|WISEA J091645.14+761451.5|     139.18812|      76.24766|      G|     579.30|       null|        null|        null|      13.414|       0.194|      13.098|       0.015|2.81e-06|  1.93e-08|
|WISEA J085617.59+731958.6|     134.07330|      73.33295|      G|     601.79|       null|        null|        null|      13.364|       0.135|      13.299|       0.012|2.78e-06|  1.71e-08|
|WISEA J090653.45+762059.4|     136.72274|      76.34984|      G|     699.37|       null|        null|        null|      13.345|       0.179|      12.327|       0.010|8.26e-07|  1.68e-08|
|WISEA J085525.06+732139.9|     133.85446|      73.36109|      G|     552.18|       null|        null|        null|      13.243|       0.122|      12.856|       0.014|2.06e-06|  1.61e-08|

Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250813k sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). 

TITLE:   GCN CIRCULAR
NUMBER:  41384
SUBJECT: LIGO/Virgo/KAGRA S250813k: Swift XRT observations, 2 X-ray sources
DATE:    25/08/15 07:26:06 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

K.L. Page (U. Leicester), P.A. Evans (U. Leicester), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto), S.B. Cenko (NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester),
A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL),
S. Campana (INAF-OAB), J.J. Delaunay (PSU), M. De Pasquale (University of Messina),
S. Dichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì (INAF-IASFPA) , V. D’Elia
(ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson University), N.
Klingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha
(NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien
(U. Leicester), M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB),
E. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:

Swift has carried out 109 observations of the LVK error region for the GW trigger
S250813k convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9),
using the 'BAYESTAR' (version BAYESTAR.fits.gz) GW localisation map. As this is a 3D
skymap, galaxy distances were taken into account in selecting which ones to observe.
The observations currently span from 2.0 ks to 164 ks after the LVK trigger, and the
XRT has covered 7.9 deg^2 on the sky (corrected for overlaps). This covers 70% of
the probability in the 'ligo-skymap-from-samples' (version
ligo-skymap-from-samples.fits.gz) skymap, and 71% after convolving with the 2MPZ
galaxy catalogue, as described by Evans et al. (2016, MNRAS,  462, 1591).

We have detected 2 X-ray sources. Each source is assigned a rank of 1-4 which
describes how likely it is to be related to the GW trigger, with 1 being the most
likely and 4 being the least likely. The ranks are described at
https://www.swift.ac.uk/ranks.php.

We have found:

  * 0 sources of rank 1
  * 0 sources of rank 2
  * 1 source of rank 3
  * 1 source of rank 4


RANK 3 sources
==============

These are uncatalogued X-ray sources, however they are not brighter than previous
upper limits, so do not stand out as likely counterparts to the GW trigger.

| Source ID	 | RA		  | Dec 	   | Err90   | Detection Flag	|
|  S250813k_X12  | 08h 54m 56.80s | +71d 23' 19.9" |	8.3" |		   GOOD |


RANK 4 sources
==============

These are catalogued X-ray sources, showing no signs of outburst compared to
previous observations, so they are not likely to be related to the GW trigger.

| Source ID	 | RA		  | Dec 	   | Err90   | Detection Flag	|
|  S250813k_X11  | 09h 17m 14.09s | +75d 30' 53.0" |	7.1" |		   GOOD |

For all flux conversions and comparisons with catalogues and upper limits from other
missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2, and photon index
(Gamma)=1.7, unless otherwise stated.

The results of the XRT automated analysis, including details of the sources listed
above, are online at https://www.swift.ac.uk/LVC/S250813k

This circular is an official product of the Swift XRT team.

TITLE:   GCN CIRCULAR
NUMBER:  41315
SUBJECT: LIGO/Virgo/KAGRA S250810ck: Identification of a GW compact binary merger candidate
DATE:    25/08/10 14:58:38 GMT
FROM:    Anna Moreso Serra at ICCUB <anna.moreso-serra@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250810ck during real-time processing of data from LIGO Hanford Observatory (H1) and Virgo Observatory (V1) at 2025-08-10 14:23:57.336 UTC (GPS time: 1438871055.336). The candidate was found by the GstLAL [1] analysis pipeline.

S250810ck is an event of interest because its false alarm rate, as estimated by the online analysis, is 5.4e-20 Hz, or about one in 1e12 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250810ck

The classification of the GW signal, in order of descending probability, is BBH (>99%), NSBH (<1%), Terrestrial (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 9%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 25 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 3454 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 628 +/- 164 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41317
SUBJECT: LIGO/Virgo/KAGRA S250810ck: Updated Sky localization and EM Bright Classification
DATE:    25/08/11 08:16:05 GMT
FROM:    Soichiro Morisaki at U. of Tokyo <soichiro.morisaki@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250810ck (GCN Circular 41315). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250810ck

Based on posterior support from parameter estimation [1], under the assumption that the candidate S250810ck is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 180 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 621 +/- 129 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe

TITLE:   GCN CIRCULAR
NUMBER:  41218
SUBJECT: LIGO/Virgo/KAGRA S250802dn: Identification of a GW compact binary merger candidate
DATE:    25/08/02 21:13:36 GMT
FROM:    nzhang352@gatech.edu

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250802dn during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2025-08-02 20:25:12.022 UTC (GPS time: 1438201530.022). The candidate was found by the cWB BBH [1], GstLAL [2], and MBTA [3] analysis pipelines.

S250802dn is an event of interest because its false alarm rate, as estimated by the online analysis, is 5.5e-08 Hz, or about one in 6 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250802dn

The classification of the GW signal, in order of descending probability, is BBH (93%), Terrestrial (7%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [4] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [4] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Three sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 41 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 4470 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2957 +/- 1075 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [4] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [5] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41220
SUBJECT: LIGO/Virgo/KAGRA S250802dn: Updated Sky localization
DATE:    25/08/03 05:18:45 GMT
FROM:    lucy.thomas@ligo.org


The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250802dn (GCN Circular 41218). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250802dn

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 3209 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3070 +/- 1219 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41179
SUBJECT: LIGO/Virgo/KAGRA S250727dc: Identification of a GW compact binary merger candidate
DATE:    25/07/27 20:24:58 GMT
FROM:    aubrey_laity@uri.edu

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250727dc during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-27 20:01:48.457 UTC (GPS time: 1437681726.457). The candidate was found by the cWB BBH [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S250727dc is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.1e-18 Hz, or about one in 1e10 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250727dc

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 23 deg^2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(06h27m, -36d17m, 3.74d, 1.98d, 29.70d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1172 +/- 256 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [5] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41187
SUBJECT: LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm
DATE:    25/07/28 15:31:25 GMT
FROM:    Paul Kuin at MSSL <npkuin@gmail.com>

N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.
Gronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall
(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of
Messina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì
(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.
Bernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.
Delaunay (PSU),  S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),
R.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri
(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha
(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DAWN/NBI and Radboud), P. O’Brien (U.
Leicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri
(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.
Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),
E. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:

Swift observed the high-probability region of LIGO/Virgo/KAGRA S250725dc
starting 65.3 minutes after the trigger date. A new UVOT source was found
in the U band with U = 19.9 +/- 0.3 mag (ABmag) image taken on 2025-07-28
00:34_55 UT at the following position:

RA = 97.47619, Dec = -37.23307 deg, or RA = 06:29:54.3,
Dec = -37:13:55.45 in sexagesimal notation.

We do not find any source on that location in DSS, using the Vizier
photometry tool, or as a nearby object known to the Minor Planet Center
MPChecker tool. The object has been registered as 2025smm by the Transient
Name Server.

A further Swift observation has been planned.

TITLE:   GCN CIRCULAR
NUMBER:  41188
SUBJECT: LIGO/Virgo/KAGRA S250727dc: Updated Sky localization
DATE:    25/07/28 17:40:55 GMT
FROM:    lucy.thomas@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250727dc (GCN Circular 41179). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250727dc

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 15 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(06h29m, -35d45m, 2.97d, 1.66d, 27.69d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1389 +/- 318 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41196
SUBJECT: LIGO/Virgo/KAGRA S250727dc: PRIME J-band upper limits of AT 2025smm
DATE:    25/07/29 18:11:07 GMT
FROM:    Joe Durbak at UMD <gcn.joedurbak@gmail.com>

LIGO/Virgo/KAGRA S250727dc : PRIME J-band upper limits of AT 2025smm

J. Durbak (UMD), O. Guiffreda (UMD), N. Passaleva, M. El Kabir, A. S. Kutyrev (NASA/GSFC), E. Troja (U Rome), S. B. Cenko (NASA/GSFC)

Following the LIGO/Virgo/KAGRA detection of S250727dc (GCN 41179), we observed a portion of the transient field in J-filter with PRIME ~32 hours after LVK detection. This field was chosen to overlap with the Swift UVOT detection of an uncatalogued source, AT 2025smm (GCN 41187).

At the position of AT 2025smm reported by Swift UVOT (GCN 41187), we detect no sources in J-band. Using nearby 2MASS stars for preliminary calibration we derive the following limiting magnitude, not corrected for Galactic extinction:  

| Filter | Mag(AB)      |
|--------|--------------|
| J      | > 19.7       |          

PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).

We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41197
SUBJECT: LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS upper limits of AT 2025smm
DATE:    25/07/29 20:55:32 GMT
FROM:    xjh@andrew.cmu.edu

Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team

DECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188).

We inspected the location of AT 2025smm, reported by Swift UVOT (GCN 41187), we find no sources in the ugri-bands. Here we report the following limiting magnitudes, not corrected for Galactic extinction:

| Filter | Mag (AB) |
|--------|----------|
| g      | > 22.15  |
| r      | > 21.46  |    
| i      | > 20.60  |             

Further analysis is underway.

We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.

TITLE:   GCN CIRCULAR
NUMBER:  41201
SUBJECT: LIGO/Virgo/KAGRA S250727dc: Swift-XRT observations, 8 X-ray sources
DATE:    25/07/30 10:45:29 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S.B. Cenko
(NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S.
Campana (INAF-OAB), J.J. Delaunay (PSU), M. De Pasquale (University of Messina), S.
Dichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì (INAF-IASFPA) , V. D’Elia
(ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson University), N.
Klingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha
(NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien
(U. Leicester), M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB),
E. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:

Swift has carried out 171 observations of the LVK error region for the GW trigger
S250727dc convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9),
using the 'bayestar.multiorder.fits,1' GW localisation map. As this is a 3D skymap,
galaxy distances were taken into account in selecting which ones to observe. The
observations currently span from 3.9 ks to 157 ks after the LVK trigger, and the XRT
has covered 11.5 deg^2 on the sky (corrected for overlaps). This covers 70% of the
probability in the imrpoved 'Bilby.offline0.multiorder.fits' skymap, and 69% after
convolving with the 2MPZ galaxy catalogue, as described by Evans et al. (2016,
MNRAS,	462, 1591).

In total, we have detected 8 X-ray sources. Each source is assigned a rank of 1-4
which describes how likely it is to be related to the GW trigger, with 1 being the
most likely and 4 being the least likely. The ranks are described at
https://www.swift.ac.uk/ranks.php.

We have found:

  * 0 sources of rank 1
  * 0 sources of rank 2 (see note)
  * 3 sources of rank 3
  * 5 sources of rank 4

NOTE: The online system reports a rank 2 source: Source S250727dc_X1. However, this
in the known blazar BL Lac (the prototype of its class of object). During our
observation, its flux was significantly increased compared to the mean value in the
LSXPS catalogue, hence the automated classification. However, it is known to be
highly variable (e.g.
https://www.swift.psu.edu/monitoring/source.php?source=BLLacertae) and the
intensities in LSXPS are drawn solely from PC-mode data so are biased towards the
lower-intensity states. We have thus reclassified this as rank 4 for this circular.


RANK 3 sources
==============

These are uncatalogued X-ray sources, however they are not brighter than previous
upper limits, so do not stand out as likely counterparts to the GW trigger.

| Source ID	 | RA		  | Dec 	   | Err90   | Detection Flag	|
|  S250727dc_X5  | 06h 23m 38.04s | -35d 38' 24.1" |	7.7" |		   GOOD |
|  S250727dc_X6  | 06h 28m 07.71s | -37d 10' 10.3" |	9.5" |	     REASONABLE |
|  S250727dc_X7  | 06h 18m 45.03s | -35d 18' 15.1" |	7.0" |		   GOOD |


RANK 4 sources
==============

These are catalogued X-ray sources, showing no signs of outburst compared to
previous observations, so they are not likely to be related to the GW trigger.

| Source ID	 | RA		  | Dec 	   | Err90   | Detection Flag	|
|  S250727dc_X1  | 06h 27m 06.55s | -35d 29' 15.0" |	3.8" |		   GOOD |
|  S250727dc_X3  | 06h 34m 35.33s | -37d 17' 12.1" |	8.3" |		   GOOD |
|  S250727dc_X8  | 06h 36m 18.37s | -37d 22' 33.4" |	4.5" |		   GOOD |
|  S250727dc_X9  | 06h 19m 53.43s | -35d 24' 28.2" |	7.7" |		   GOOD |
| S250727dc_X10  | 06h 20m 08.64s | -34d 37' 54.4" |	5.8" |		   GOOD |


In addition, we took a 1.9 ks exposure of the UVOT source reported by Kuin et al.
(GCN Circ. 41187). No XRT counterpart was found, with a 3-sigma upper limit of
3.9e-3 ct/sec (approx 1.6e-13 erg cm^-s s^-1, 0.3-10 keV).

The results of the XRT automated analysis, including details of the sources listed
above, are online at https://www.swift.ac.uk/LVC/S250727dc

This circular is an official product of the Swift XRT team.

TITLE:   GCN CIRCULAR
NUMBER:  41202
SUBJECT: LIGO/Virgo/KAGRA S250727dc: Upper limits from Swift/BAT-GUANO
DATE:    25/07/30 17:49:32 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman, James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 100.00% of the GW localization probability ([Bilby.offline0.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250727dc/files/Bilby.offline0.multiorder.fits)) at merger time. A fraction 100.00% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |0.56  |0.87  |0.79  |0.92  
|1.024        |0.28  |0.44  |0.40  |0.47  
|4.096        |0.15  |0.24  |0.22  |0.25  
|16.384       |0.10  |0.15  |0.14  |0.16  


The upper limits as function of sky position are plotted here, alongside the GW localization: https://zenodo.org/records/16612515

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

TITLE:   GCN CIRCULAR
NUMBER:  41205
SUBJECT: LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm
DATE:    25/07/30 20:28:25 GMT
FROM:    Paul Kuin at MSSL <npkuin@gmail.com>

N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.
Gronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall
(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of
Messina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì
(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.
Bernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.
Delaunay (PSU),  S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),
R.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri
(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha
(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DAWN/NBI and Radboud), P. O’Brien (U.
Leicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri
(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.
Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),
E. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:

Swift re-observed the location of AT 2025smm in the high-probability region
of LIGO/Virgo/KAGRA S250725dc from 1.56 to 1.63 d after the trigger. The
source is no longer detected. we obtained the following 3-sigma upper
limits:

Filter Exposure(s) 3-sigma UL AB mag)
UVW2     606        22.3
UVM2     466        21.8
UVW1     303        21.5
   U     151        20.8
   B     151        20.1
   V     151        19.3

No corrections were made for interstellar extinction.

TITLE:   GCN CIRCULAR
NUMBER:  41211
SUBJECT: LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates
DATE:    25/07/31 13:19:11 GMT
FROM:    Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>

Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Nora Sherman (Boston U.), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, H. Thomas Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 2025-07-29 09:39:10 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250727dc (GCN 41179). Observations covered the 90% localization region of the event (Prop ID: 2023B-851374; PI Andreoni/Palmese).

We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. We require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, Quaia, and LQAC-6 AGN catalogs (Secrest et al. 2015, Flesch 2023, Storey-Fisher et al. 2024, Souchay et al. 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.

After candidate selection we report the 4 high confidence candidates listed below, including 2 nuclear candidates (likely AGN), and 1 likely supernova. Additionally, the transient AT2025ssi is the probable AGN Quaia_1070591 with a measured redshift of z~0.24 +- 0.08, which falls within the reported LVK volume. Further observations are ongoing, and we encourage followup of the 4 candidates identified herein.

| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |
|----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------|
| SN_LIKE | 3334740 | AT2025ssf | 99.02238 | -36.026584 | 20.65 | 0.05 | 20.32 | 0.03 | N/A  | N/A  | 21.21 | 0.12 |
| AGN_LIKE | 3334614 | AT2025ssg | 97.813775 | -36.075323 | 21.06 | 0.1 | 20.58 | 0.07 | N/A  | N/A  | 19.99 | 0.04 |
| AGN_LIKE | 3334106 | AT2025ssh | 97.339644 | -36.600508 | 21.79 | 0.21 | N/A  | N/A  | 21.08 | 0.09 | 19.5 | 0.04 |
| AGN_LIKE | 3334650 | AT2025ssi | 99.146548 | -37.499065 | 20.09 | 0.05 | N/A  | N/A  | 19.68 | 0.03 | N/A | N/A  |

Additionally, we also recover two of the candidates reported by SWIFT-XRT (GCN 41201). One is the known blazar BL Lacertae (S250727dc_X1), which has been mentioned to be highly variable. The other (S250727dc_X7) is the galaxy ESO 365-5. Both of these galaxies have been spectroscopically measured to have redshifts z=0.055 (García-Pérez et al. 2024) and z=0.045 (Jones et al. 2009) respectively, which both lie outside of the LVK localization volume.

| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |
|----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------|
| AGN_LIKE | 3334338 | S250727dc_X1 | 96.778098 | -35.487631 | 17.32 | 0.01 | 16.98 | 0.01 | N/A  | N/A  | 16.5 | 0.0 |
| AGN_LIKE | 3334206 | S250727dc_X7 | 94.688512 | -35.304098 | 18.79 | 0.02 | 18.32 | 0.02 | 18.34 | 0.01 | 18.51 | 0.02 |

We do not recover any of the other candidates reported by SWIFT-XRT, nor the transient AT2025smm reported by SWIFT-UVOT (GCN 41187).

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41214
SUBJECT: LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates
DATE:    25/07/31 20:21:13 GMT
FROM:    Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>

Tomás Cabrera (CMU), Xander J. Hall (CMU), Lei Hu (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team

DECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188).

We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and visually inspect the remaining transients.  All candidates included in this GCN have been reported to the TNS.  We report the novel candidates that pass our cuts at this stage in the first table; in the following sections we present genuine transients we reject as gravitational wave counterparts, and the confirmation of some previously reported candidates.

We report the discovery of the following 3 nuclear transients that we do not exclude from our search.  For AT2025stf, the nucleus of the host is ambiguous, and so it is possible this transient is non-nuclear, which would eliminate it as a favored candidate.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |
| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |
| T202507290634429m374411 | AT 2025ssz | 98.678750 | -37.736389 | 2025-07-29 09:49:00.192 | 21.476 | 0.140 | 0.487 |
| T202507290636497m363725 | AT 2025ste | 99.207083 | -36.623611 | 2025-07-29 09:49:00.192 | 22.437 | 0.408 | 1.627 |
| T202507290626025m372843 | AT 2025stf | 96.510373 | -37.478478 | 2025-07-29 09:47:11.817 | 21.193 | 0.073 | -0.211 |

The following transients are coincident with objects in the quaia and Milliquas catalogs, and have redshifts that place them outside of the 90% localization volume for the gravitational wave event.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | possible_host | host_z |
| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | ----------- | --------- |
| T202507290621597m351731 | AT 2025ssx | 95.498727 | -35.291956 | 2025-07-29 10:26:17.504 | 20.945 | 0.061 | 0.661 | quaia 2891332298170436480 | 0.47 |
| T202507290634416m350638 | AT 2025sta | 98.673141 | -35.110434 | 2025-07-29 09:52:36.137 | 19.783 | 0.047 | 0.288 | quaia 5581689916779289344 | 0.53 |
| T202507290629125m373923 | AT 2025std | 97.302216 | -37.656276 | 2025-07-29 09:49:00.617 | 19.945 | 0.035 | 0.819 | milliquas 6dF J062912.5-373922 | 0.06 |

The following transients are nuclear, but are hosted by nearby galaxies (as determined by their large angular sizes), and so are unlikely to be associated with the gravitational wave event.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |
| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |
| T202507290623386m353830 | AT 2025ssy | 95.910743 | -35.641755 | 2025-07-29 09:45:22.413 | 19.100 | 0.013 | 0.454 |
| T202507290627447m354045 | AT 2025stg | 96.936244 | -35.679174 | 2025-07-29 09:58:01.892 | 20.724 | 0.047 | 0.916 |
| T202507290618452m351815 | AT 2025sth | 94.688504 | -35.304203 | 2025-07-29 09:45:22.413 | 19.080 | 0.014 | 0.441 |
| T202507290627067m352915 | AT 2025sti | 96.778033 | -35.487632 | 2025-07-29 09:58:01.892 | 17.192 | 0.004 | 0.622 |

The following transients are non-nuclear, and appear to be supernovae.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |
| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |
| T202507290626444m373046 | AT 2025stb | 96.684801 | -37.512899 | 2025-07-29 09:47:11.817 | 21.174 | 0.072 | 0.028 |
| T202507290625106m332028 | AT 2025stc | 96.294042 | -33.341084 | 2025-07-29 09:56:12.517 | 20.252 | 0.056 | -0.166 |
       
We additionally confirm the following candidates initially reported in GCN 41211, including the nuclear candidate with a Quaia host at a favorable redshift (AT 2025ssi).

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |
| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |
| T202507290636054m360136 | AT 2025ssf | 99.022404 | -36.026624 | 2025-07-29 10:23:00.741 | 20.624 | 0.040 | [no mag_i] |
| T202507290631153m360431 | AT 2025ssg | 97.813758 | -36.075317 | 2025-07-29 10:23:00.741 | 20.412 | 0.028 | [no mag_i] |
| T202507290636352m372956 | AT 2025ssi | 99.146615 | -37.499005 | 2025-07-29 09:49:00.617 | 20.112 | 0.047 | 0.391 |

Further analysis is underway.

We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.

TITLE:   GCN CIRCULAR
NUMBER:  41216
SUBJECT: LIGO/Virgo/KAGRA S250727dc: GRANDMA/OPD  observations of AT2025smm
DATE:    25/08/01 22:47:59 GMT
FROM:    Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>

Wagner Corradi (LNA), Nélio Sasaki (UEA-Parintins), Leandro de Almeida (LNA), Felipe Navarete (LNA), S. Antier (OCA/IJCLAB), Yodgor Rajabov (UBAI), Yusufjion Tillayev (UBAI), Eslam Elhosseiny (NRIAG), C. Andrade (UMN), D. Turpin (CEA), I. Tosta e Melo (IFN), M. Coughlin (UMN), P. Hello (IJCLAB), P-A. Duverne (APC), S. Karpov (FZU) and T. Pradier (IPHC)   on behalf of Observatório do Pico dos Dias (OPD/LNA/Brazil) and the GRANDMA collaboration, we inspected the optical candidate AT2025smm (GCNs 41179) of the gravitational-wave event  S250727dc (GCNs 41179, 41487), reported by Swift/UVOT (GCN 41179,GCN 41187, GCN 41188, GCN 41196, GCN 41197, GCN 41201, GCN 41202, GCN 41205, GCN 41211 and GCN 41214). Observations were conducted with the 0.6 m telescope at OPD, obtaining six frames of 300s exposures in the Bessel-R band between 2025-07-31T08:18:35.268  and 2025-07-31T08:48:35.268 UTC. No source is detected at the reported position in the stacked image. The final stacked frame covers a field of view of 11.6′ × 11.6′ with a pixel scale of 0.34″ arcsecond per pixel.
Here we report the following limiting magnitudes, uncorrected for Galactic extinction:
Filter | Mag (AB)
     r | > 22.53
This result is in agreement with the analysis of DECam GW-MMADS (GCN 41197).
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). GRANDMA is a worldwide telescope network (http://grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518).

TITLE:   GCN CIRCULAR
NUMBER:  41178
SUBJECT: LIGO/Virgo/KAGRA S250727cl: Identification of a GW compact binary merger candidate
DATE:    25/07/27 19:02:50 GMT
FROM:    aubrey_laity@uri.edu

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250727cl during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-27 18:03:29.655 UTC (GPS time: 1437674627.655). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], MLy [5], PyCBC Live [6], and SPIIR [7] analysis pipelines.

S250727cl is an event of interest because its false alarm rate, as estimated by the online analysis, is 4e-20 Hz, or about one in 1e12 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250727cl

The classification of the GW signal, in order of descending probability, is BBH (>99%), BNS (<1%), NSBH (<1%), or Terrestrial (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 31 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 379 deg^2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1817 +/- 454 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034
 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41183
SUBJECT: LIGO/Virgo/KAGRA S250727cl: Updated Sky localization
DATE:    25/07/28 09:51:17 GMT
FROM:    Elise Sänger at Max Planck Institute for Gravitational Physics (AEI Potsdam) <elise.sanger@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250727cl (GCN Circular 41178). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250727cl

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 205 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2152 +/- 515 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41171
SUBJECT: LIGO/Virgo/KAGRA S250726ak: Identification of a GW compact binary merger candidate
DATE:    25/07/26 09:16:48 GMT
FROM:    Cassius Melo at Universidade Federal de Alfenas - UNIFAL-MG <cassiusanderson.melo@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250726ak during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-26 08:28:15.370 UTC (GPS time: 1437553713.370). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S250726ak is an event of interest because its false alarm rate, as estimated by the online analysis, is 4.7e-12 Hz, or about one in 1e4 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250726ak

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1027 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3384 +/- 1067 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41174
SUBJECT: LIGO/Virgo/KAGRA S250726ak: Updated Sky localization
DATE:    25/07/27 10:32:41 GMT
FROM:    Lorenzo Pompili at Max Planck Institute for Gravitational Physics <lorenzo.pompili@aei.mpg.de>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250726ak (GCN Circular 41171). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250726ak

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 361 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3333 +/- 1448 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41204
SUBJECT: LIGO/Virgo/KAGRA S250726ak: Upper limits from Swift/BAT-GUANO
DATE:    25/07/30 17:57:00 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman, James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 99.9% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250726ak/files/Bilby.multiorder.fits)) at merger time. A fraction 40.74% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |12.10 |7.77  |6.83  |8.74  
|1.024        |6.19  |3.97  |3.49  |4.46  
|4.096        |3.34  |2.15  |1.89  |2.41  
|16.384       |2.10  |1.35  |1.18  |1.51  


The upper limits as function of sky position are plotted here, alongside the GW localization: https://zenodo.org/records/16614782

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

TITLE:   GCN CIRCULAR
NUMBER:  41155
SUBJECT: LIGO/Virgo/KAGRA S250725l: Identification of a GW compact binary merger candidate
DATE:    25/07/25 04:53:42 GMT
FROM:    Kiet Pham <kiet.pham@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250725l during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-25 04:20:50.940 UTC (GPS time: 1437452468.940). The candidate was found by the GstLAL [1], MBTA [2], and PyCBC Live [3] analysis pipelines.

S250725l is an event of interest because its false alarm rate, as estimated by the online analysis, is 3e-08 Hz, or about one in 1 year, 23 days. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250725l

The classification of the GW signal, in order of descending probability, is BBH (99%), Terrestrial (1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [4] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [4] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (11.0, 22.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 31 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1990 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2186 +/- 673 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [3] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [4] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [5] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41156
SUBJECT: LIGO/Virgo/KAGRA S250725l: Identification of a GW compact binary merger candidate (duplicate submission)
DATE:    25/07/25 04:54:50 GMT
FROM:    Junjie Zhao at Henan Academy of Sciences <junjiezhao@hnas.ac.cn>

This is a duplicate of GCN Circular 41155.

TITLE:   GCN CIRCULAR
NUMBER:  41175
SUBJECT: LIGO/Virgo/KAGRA S250725l: Updated Sky localization
DATE:    25/07/27 11:06:08 GMT
FROM:    Lorenzo Pompili at Max Planck Institute for Gravitational Physics <lorenzo.pompili@aei.mpg.de>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250725l (GCN Circular 41155). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250725l

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 1532 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2567 +/- 766 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41203
SUBJECT: LIGO/Virgo/KAGRA S250725l: Upper limits from Swift/BAT-GUANO
DATE:    25/07/30 17:54:00 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman, James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 100.0% of the GW localization probability ([Bilby.offline0.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250725l/files/Bilby.offline0.multiorder.fits)) at merger time. A fraction 38.64% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |9.96  |6.98  |6.25  |7.72  
|1.024        |5.08  |3.56  |3.19  |3.94  
|4.096        |2.74  |1.92  |1.72  |2.13  
|16.384       |1.71  |1.20  |1.08  |1.33  


The upper limits as function of sky position are plotted here, alongside the GW localization: https://zenodo.org/records/16614280

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

TITLE:   GCN CIRCULAR
NUMBER:  41154
SUBJECT:  LIGO/Virgo/KAGRA S250725j: Identification of a GW compact binary merger candidate
DATE:    25/07/25 04:48:19 GMT
FROM:    Hanlin Song and Junjie Zhao at Peking University and Henan Academy of Sciences <hanlin@stu.pku.edu.cn>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250725j during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-25 04:09:44.252 UTC (GPS time: 1437451802.252). The candidate was found by the GstLAL [1] analysis pipeline.

S250725j is an event of interest because its false alarm rate, as estimated by the online analysis, is 4.3e-11 Hz, or about one in 1e3 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250725j

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (11.0, 22.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 22 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 28 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(15h34m, -35d25m, 3.97d, 2.25d, 153.93d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 478 +/- 107 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41165
SUBJECT: LIGO/Virgo/KAGRA S250725j: NED Galaxies in the 3-Initial Localization Volume
DATE:    25/07/25 15:34:28 GMT
FROM:    David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>

David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)

On behalf of the NASA/IPAC Extragalactic Database (NED) Team.

We spatially cross-matched the LVK S250725j-3-Initial sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 594 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:

Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250725j/3                       
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250725j/3/20

The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.

|                  objname|            ra|           dec|objtype|    DistMpc|DistMpc_unc|  m_NUV|m_NUV_unc|        m_Ks|    m_Ks_unc|        m_W1|    m_W1_unc|    P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|-------|---------|------------|------------|------------|------------|--------|----------|
|WISEA J152748.68-355739.1|     231.95286|     -35.96088|      G|     452.82|       null|   null|     null|      13.128|       0.132|      10.686|       0.006|4.47e-06|  4.56e-08|
|WISEA J153844.18-344009.1|     234.68409|     -34.66922|      G|     480.22|       null|   null|     null|      12.995|       0.143|      10.976|       0.010|3.62e-06|  3.52e-08|
|WISEA J153525.42-344743.9|     233.85592|     -34.79554|      G|     524.15|       null|   null|     null|      13.715|       0.214|      11.189|       0.018|3.34e-06|  2.91e-08|
|WISEA J152653.58-355046.3|     231.72329|     -35.84620|      G|     431.02|       null|   null|     null|      13.223|       0.157|      10.966|       0.009|4.06e-06|  2.89e-08|
|WISEA J154628.27-340002.3|     236.61826|     -34.00070|      G|     425.21|       null|   null|     null|      13.479|       0.169|       9.563|       0.006|1.10e-06|  2.79e-08|
|WISEA J153024.70-351033.1|     232.60293|     -35.17588|      G|     529.76|       null|   null|     null|      12.578|       0.113|      11.182|       0.008|2.90e-06|  2.58e-08|
|WISEA J153324.20-354028.6|     233.35085|     -35.67462|      G|     435.53|       null|   null|     null|      13.377|       0.171|      11.600|       0.011|5.94e-06|  2.45e-08|
|WISEA J153416.18-351536.7|     233.56786|     -35.26055|      G|     415.65|       null|   null|     null|      13.472|       0.230|      11.551|       0.011|5.56e-06|  2.18e-08|
|WISEA J153550.83-352815.7|     233.96181|     -35.47105|      G|     452.48|       null|   null|     null|      12.155|       0.114|      11.835|       0.011|5.59e-06|  2.01e-08|
|WISEA J152918.46-355636.4|     232.32692|     -35.94346|      G|     469.28|       null|   null|     null|      13.745|       0.216|      11.703|       0.011|4.59e-06|  1.97e-08|
|WISEA J153406.90-352554.3|     233.52878|     -35.43177|      G|     444.26|       0.49|   null|     null|      12.177|       0.090|      12.031|       0.023|5.84e-06|  1.68e-08|
|WISEA J153130.18-350639.3|     232.87576|     -35.11092|      G|     419.28|       0.79|   null|     null|      12.607|       0.111|      11.785|       0.011|4.73e-06|  1.49e-08|
|WISEA J153144.43-352246.2|     232.93514|     -35.37951|      G|     474.13|       null|   null|     null|      12.991|       0.145|      12.199|       0.016|4.99e-06|  1.38e-08|
|WISEA J153031.02-350955.4|     232.62929|     -35.16539|      G|     478.85|       null|   null|     null|      13.515|       0.194|      12.117|       0.015|4.17e-06|  1.28e-08|
|WISEA J153512.75-350712.9|     233.80314|     -35.12027|      G|     470.25|       null|   null|     null|      13.554|       0.177|      12.400|       0.011|5.07e-06|  1.18e-08|
|WISEA J152932.42-350115.2|     232.38508|     -35.02090|      G|     538.67|       null|   null|     null|      13.523|       0.198|      11.824|       0.007|2.30e-06|  1.16e-08|
|WISEA J154625.51-332529.0|     236.60631|     -33.42475|      G|     445.85|       null|   null|     null|      12.552|       0.113|      10.145|       0.007|6.91e-07|  1.13e-08|
|WISEA J153354.06-351626.2|     233.47529|     -35.27397|      G|     397.36|       null|   null|     null|      13.103|       0.132|      12.174|       0.012|5.35e-06|  1.08e-08|
|WISEA J153959.24-350547.6|     234.99684|     -35.09658|      G|     378.08|       null|   null|     null|      13.710|       0.194|      11.652|       0.010|3.45e-06|  1.05e-08|
|WISEA J153052.19-345946.7|     232.71748|     -34.99631|      G|     454.26|       0.79|   null|     null|      12.909|       0.128|      12.253|       0.014|4.22e-06|  1.01e-08|

Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250725j sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). 

TITLE:   GCN CIRCULAR
NUMBER:  41168
SUBJECT: LIGO/Virgo/KAGRA S250725j: Updated Sky localization
DATE:    25/07/25 17:04:56 GMT
FROM:    Lorenzo Pompili at Max Planck Institute for Gravitational Physics <lorenzo.pompili@aei.mpg.de>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250725j (GCN Circular 41154). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250725j

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 19 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(15h32m, -36d06m, 3.23d, 1.84d, 158.46d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 380 +/- 93 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41177
SUBJECT: LIGO/Virgo/KAGRA S250725j: DECam GW-MMADS candidates
DATE:    25/07/27 18:06:49 GMT
FROM:    xjh@andrew.cmu.edu

Xander Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team

DECam observed the southern high probability area of the LVK gravitational wave candidate S250725j (GCN 41154) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-07-25 23:13:43 UTC (Prop ID: 2025A-227091; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41168).

We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, visually inspect the remaining transients, and remove transients showing SNR>5 detections predating the gravitational wave alert from ATLAS forced photometry. We reported on TNS new transients within the LVK 99% CI area, and we report here those matched to a NED galaxy (GCN 41165):

| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_r | NED separation |
| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- |
| T202507251524174m373721 | AT 2025shz | 231.072434 |  -37.622634 | 2025-07-25 23:29:54.630 | 22.63 | 0.06 | 0.37 | 17.0”
| T202507251542117m350010* | AT 2025sib | 235.548892 |  -35.002876 | 2025-07-25 23:15:43.009 | 22.5 | 0.1 | 0.7 | 2.1”
| T202507251538092m361517 | AT 2025sia | 234.538140 |  -36.254760 | 2025-07-25 23:19:38.393 | 21.74 | 0.03 | 1.06 | 1.1”

\* - *We note that these transients may have tentative low SNR pre-detections from ATLAS forced photometry.*

We note that AT 2025shz cannot be excluded as a variable star and that it shows an extremely quick rise in the rz filters between two observations in the same night with a rise of several mag/day. We also note that AT 2025sib, shows a gradual decline in the grz filters between two observations with an average decline of ~0.5 mag/day.

Further analysis is underway.

We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.

TITLE:   GCN CIRCULAR
NUMBER:  41206
SUBJECT: LIGO/Virgo/KAGRA S250725j: DECam DESGW Epoch 1-2 Candidates
DATE:    25/07/31 00:52:54 GMT
FROM:    Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>

Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Nora Sherman (Boston U.), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, Tom Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 2025-07-25 23:13:43 UTC and 2025-07-28 23:13:54 UTC, the Dark Energy Camera (DECam) began the first and second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250725j (GCN 41154). We observed nine fields centered on the following ICRS coordinates:
| RA | DEC |
|----------|----------|
| 231.833836 | -37.23415|
|234.585764 | -35.2584|
|232.520774 | -35.64614|
|233.935432 | -36.84845|
|236.026051 | -36.42542|
|230.4476 | -35.99735|
|233.180871 | -34.0649|
|236.640727 | -34.83446|
|229.723284 | -37.58219|

These pointings cover the 90% localization region of candidate gravitational-wave event S250725j.

All fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 23.2 in g, 22.9 in r, and 22.6 in i, and 21.6 in z. The limiting magnitudes achieved for epoch 2 were 22.5 in g, 22.6 in r, and 22.3 in i, and 21.6 in z.

We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.

After candidate selection we report the 3 high confidence candidates listed below, including 2 nuclear candidates (likely active galactic nuclei) and 1 likely supernova. Further observations are ongoing, and we encourage followup of the 3 candidates identified herein.

| TYPE     	| ID      	| ATNAME 	| RA         	| DEC       	| MAG_G 	| MAG_G_ERR 	| MAG_R 	| MAG_R_ERR 	| MAG_I 	| MAG_I_ERR 	| MAG_Z 	| MAG_Z_ERR 	|
|----------	|---------	|--------	|------------	|-----------	|-------	|-----------	|-------	|-----------	|-------	|-----------	|-------	|-----------	|
| SN_LIKE | 3141695 | AT2025srk | 234.755524 | -36.425993 | 21.51 | 0.02 | 21.0 | 0.02 | 21.27 | 0.03 | 22.06 | 0.27 |
| AGN_LIKE | 3237805 | AT2025srl | 234.90473 | -36.726032 | 23.55 | 0.19 | N/A  | N/A  | 19.74 | 0.01 | N/A | N/A  |
| AGN_LIKE | 3141486 | AT2025srm | 232.987656 | -36.816564 | 22.43 | 0.06 | 22.07 | 0.04 | 22.74 | 0.17 | 21.21 | 0.05 |

Additionally, we also recover two of the candidates reported by GW-MMADS (GCN 41177). The candidate AT2025sib is of particular note, as its photometric redshift of z=0.07 reported by 2MASS (Bilicki et al 2014) lies within the LVK 90% volume.

| TYPE     	| ID      	| ATNAME 	| RA         	| DEC       	| MAG_G 	| MAG_G_ERR 	| MAG_R 	| MAG_R_ERR 	| MAG_I 	| MAG_I_ERR 	| MAG_Z 	| MAG_Z_ERR 	|
|----------	|---------	|--------	|------------	|-----------	|-------	|-----------	|-------	|-----------	|-------	|-----------	|-------	|-----------	|
| AGN_LIKE | 3141729 | AT2025shz | 231.072453 | -37.6226 | 22.77 | 0.08 | 22.36 | 0.08 | N/A  | N/A  | N/A | N/A  |
| SN_LIKE | 3141898 | AT2025sib | 235.548953 | -35.002893 | 21.72 | 0.04 | 20.7 | 0.02 | N/A  | N/A  | 20.72 | 0.07 |

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.

TITLE:   GCN CIRCULAR
NUMBER:  41213
SUBJECT: LIGO/Virgo/KAGRA S250725j: GOTO pre-trigger detection of AT2025sib
DATE:    25/07/31 14:27:23 GMT
FROM:    Nusrin Habeeb at University of Leicester <nh312@leicester.ac.uk>


B. Godson, N. Habeeb, K. Ackley, D. O’Neill, A. Kumar, G. Ramsay, M. J. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration: 

We report on historical observations of counterpart candidate AT 2025sib (Hall et al. GCN 41177, McMahon et al. GCN 41206) with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024).

We find evidence of emission of the source starting at 2025-07-13 08:51 with a 3-sigma observation of L = 20.2 +/- 0.3 AB mag, with our final observation on 2025-07-21 10:11 with L = 20.8 +/- 0.3 AB mag at 3-sigma. We cannot rule out the variable nature of this source and find no evidence of the decay reported in Hall et al. GCN 41177 at these early times. We note that the source appears coincident with an object in the 2MASS Point Source Catalog, 2MASS15421160-3500115 (Cutri et al 2003).

GOTO last covered the field of S250725j serendipitously during survey mode on 2025-07-21. We did not perform follow-up for S250725j due to poor weather conditions at GOTO-South. In addition, we find no evidence of pre-detection emission for the other sources reported in GCN 41177, GCN 41206.

Continued analysis of additional candidates is ongoing.

Difference imaging was performed using deeper template observations. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. 

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

TITLE:   GCN CIRCULAR
NUMBER:  41595
SUBJECT: LIGO/Virgo/KAGRA S250725j: Observations with the NSF-DOE Vera C. Rubin Observatory
DATE:    25/08/29 19:02:14 GMT
FROM:    Sean MacBride at University of Zurich <sean.p.macbride@gmail.com>

Sean MacBride (University of Zurich), Shreya Anand (Stanford University), Erin Howard, Ian Sullivan, David Wang, Lynne Jones, Peter Yoachim, Eric Bellm (University of Washington), Bob Armstrong (Lawrence Livermore National Laboratory), Michael Wood-Vasey (University of Pittsburgh), Alex Drlica-Wagner, Ken Herner (Fermi National Accelerator Laboratory), Gautham Narayan (University of Illinois), Tatiana Acero Cuellar, Federica Bianco (University of Delaware), Bruno Sánchez (Aix Marseille Université), John Banovetz (Lawrence Berkeley National Laboratory), Kris Mortensen, Paulina Venegas, Yijung Kang, Yousuke Utsumi, and Tiago Ribeiro (NSF-DOE Vera C Rubin Observatory), reporting on behalf of NSF-DOE Vera C Rubin Observatory:

We observed the localization region of the binary black hole merger S250725j, reported by LIGO/Virgo/KAGRA (GCN 41154, GCN 41168) with the 9.6 square degree field of view LSST Camera mounted on the 8.4-m Simonyi Survey Telescope at the Vera C. Rubin Observatory. This is the first Target-of-Opportunity observation by Rubin Observatory.

Rubin Observatory is conducting a Science Validation (SV) survey as part of its commissioning activities [1]. During this period, data quality and acquisition rate are variable. Not all data products that will be available during the Legacy Survey of Space and Time (LSST) operations are currently available [2]. 

Observations were conducted as a part of commissioning the Rubin Target-of-Opportunity (ToO) system. We used the SCiMMA Hopskotch infrastructure to send the alert data from LVK to Rubin Observatory infrastructure [3]. The observing strategy for this gravitational wave event follows the community recommendation for a binary black-hole merger candidate of this distance and sky condition [4]. The observing strategy calls for multiple epochs of observation, but weather conditions have prevented further observations.

The localization region of S250725j is not inside the SV survey footprint, so no prior Rubin images of this region were available. We obtained two visits in each of the u, r, and i bands with 30s exposures. Observations were taken on the night of July 29, 2025, beginning at 02:28:07.875 UTC and concluding at 03:20:18.800 UTC. We observed with nine 9.6 square degree pointings centered on the following ICRS coordinates:

| RA      | DEC     |
|---------|---------|
| 230.604 | -38.402 |
| 232.198 | -35.510 |
| 232.568 | -41.024 |
| 233.614 | -32.596 |
| 234.198 | -38.132 |
| 235.633 | -35.209 |
| 236.925 | -32.268 |
| 237.769 | -37.776 |

We covered 94% of the total localization area in all three bands, reaching a median depth of 24.1 mag in r band, 23.8 mag in the i band, and 23.6 in u band. 

Due to the lack of prior Rubin observations of the region, we performed image differencing against DECam templates from DES DR2 [5]. To support the usage of DECam templates, we devised a custom image processing task to create DECam templates from DES templates for use with LSSTCam observations. Apart from the DECam template task, we utilize the standard LSST data release production pipeline to obtain difference images and photometry on difference image sources [6]. 

With only a single epoch of observations at Rubin observatory, we are unable to characterize viable candidates from Rubin observations alone. Here, we report on candidates reported in GCN 41206 and GCN 41177.

We detect four candidates reported by the DESGW collaboration (GCN 41206), and two candidates reported by the GW-MMADS collaboration (GCN 41177, two candidates recovered by both collaborations). AT2025sib, AT2025srk, and AT2025srm are detected in both r and i band difference images. At the position of AT2025srm and AT2025sib there is a galaxy in the DES template images, and a point source present in the difference images. AT2025srk shows a point source present in the LSSTCam images, but not the DES templates. We caution that AT2025shz may be a variable star, as there is a point source at the location in the DES templates.

| AT name   | Observation Date | RA        | Dec        | r     | unc_r | i     | unc_i |
|-----------|------------------|-----------|------------|-------|-------|-------|-------|
| AT2025sib | 2025-07-29       | 235.548892 | -35.002876 | 20.95 | 0.03  | 20.76 | 0.03  |
| AT2025srk | 2025-07-29       | 234.755524 | -36.425993 | 21.17 | 0.04  | 21.31 | 0.06  |
| AT2025srm | 2025-07-29       | 232.987656 | -36.816564 | 21.90 | 0.05  | 22.99 | 0.21  |
| AT2025shz | 2025-07-29       | 231.072434 | -37.622634 | 22.25 | 0.07  | 21.97 | 0.11  |

Analysis of these candidates is ongoing.

This material is based upon work supported in part by the National Science Foundation through Cooperative Agreements AST-1258333 and AST-2241526 and Cooperative Support Agreements AST-1202910 and 2211468 managed by the Association of Universities for Research in Astronomy (AURA), and the Department of Energy under Contract No. DE-AC02-76SF00515 with the SLAC National Accelerator Laboratory managed by Stanford University. Additional Rubin Observatory funding comes from private donations, grants to universities, and in-kind support from LSST-DA Institutional Members.

[1] https://sitcomtn-005.lsst.io, §6

[2] https://lse-163.lsst.io 

[3] www.scimma.org

[4] www.inspirehep.net/literature/2846480 

[5] www.inspirehep.net/literature/1841103

[6] https://pstn-019.lsst.io

TITLE:   GCN CIRCULAR
NUMBER:  41075
SUBJECT: LIGO/Virgo/KAGRA S250712cd: Identification of a GW compact binary merger candidate
DATE:    25/07/12 15:04:55 GMT
FROM:    Cassius Melo at Universidade Federal de Alfenas - UNIFAL-MG <cassiusanderson.melo@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250712cd during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-12 14:25:32.329 UTC (GPS time: 1436365550.329). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], and MBTA [4] analysis pipelines.

S250712cd is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.7e-11 Hz, or about one in 1e3 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250712cd

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 28 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is well fit by an ellipse with an area of 395 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(19h30m, +55d32m, 15.21d, 8.35d, 104.26d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2177 +/- 664 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [6] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41080
SUBJECT: LIGO/Virgo/KAGRA S250712cd: Updated Sky localization
DATE:    25/07/13 19:39:42 GMT
FROM:    Sylvia Biscoveanu at Northwestern CIERA <sylvia.biscoveanu@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250712cd (GCN Circular 41075). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250712cd

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 180 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(19h24m, +55d04m, 10.93d, 5.26d, 101.96d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2439 +/- 870 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41062
SUBJECT: LIGO/Virgo/KAGRA S250711q: Identification of a GW compact binary merger candidate
DATE:    25/07/11 04:36:24 GMT
FROM:    Jyotirmaya Mohanta at University of Tsukuba  <s2430161@u.tsukuba.ac.jp>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250711q during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-11 03:27:25.108 UTC (GPS time: 1436239663.108). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], and MBTA [4] analysis pipelines.

S250711q is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.6e-09 Hz, or about one in 20 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250711q

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 31 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 2964 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 9738 +/- 2925 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [6] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41120
SUBJECT: LIGO/Virgo/KAGRA S250711q: Upper limits from Swift/BAT-GUANO
DATE:    25/07/18 19:06:21 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman, James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 73.58% of the GW localization probability ([bayestar.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250711q/files/bayestar.multiorder.fits)) at merger time. A fraction 58.56% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |3.27  |2.72  |2.52  |2.89  
|1.024        |1.67  |1.39  |1.29  |1.48  
|4.096        |0.90  |0.75  |0.70  |0.80  
|16.384       |0.57  |0.47  |0.44  |0.50  


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://zenodo.org/records/16106142

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

TITLE:   GCN CIRCULAR
NUMBER:  41132
SUBJECT: LIGO/Virgo/KAGRA S250711q: Updated Sky localization
DATE:    25/07/21 21:59:28 GMT
FROM:    rudall@caltech.edu

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further offline analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250711q (GCN Circular 41062). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline1.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250711q

For the Bilby.offline1.multiorder.fits,0 sky map, the 90% credible region is 2016 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4756 +/- 2023 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  40980
SUBJECT: LIGO/Virgo/KAGRA S250705cb: Identification of a GW compact binary merger candidate
DATE:    25/07/05 16:58:35 GMT
FROM:    Mayara Hilgert Pacheco at LIGO <mayara.pacheco@inpe.br>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250705cb during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-05 16:29:56.842 UTC (GPS time: 1435768214.842). The candidate was found by the GstLAL [1], MBTA [2], and PyCBC Live [3] analysis pipelines.

S250705cb is an event of interest because its false alarm rate, as estimated by the online analysis, is 9.5e-10 Hz, or about one in 33 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250705cb

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [4] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [4] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 27 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 510 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1209 +/- 336 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [3] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [4] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [5] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41034
SUBJECT: LIGO/Virgo/KAGRA S250705cb: Updated Sky localization
DATE:    25/07/08 16:03:00 GMT
FROM:    lucy.thomas@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250705cb (GCN Circular 40980). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250705cb

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 333 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1143 +/- 299 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  40935
SUBJECT: LIGO/Virgo/KAGRA S250704ab: Identification of a GW compact binary merger candidate
DATE:    25/07/04 05:24:59 GMT
FROM:    Pan Guo at KAGRA <panguocas@gmail.com>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250704ab during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-04 04:30:48.040 UTC (GPS time: 1435638666.040). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], and PyCBC Live [5] analysis pipelines.

S250704ab is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250704ab

The initial classification of the GW signal, in order of descending probability, based on chirp-mass information only [5], is BBH (82%), NSBH (17%), Terrestrial (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 14%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 27 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 77 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 536 +/- 124 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  40937
SUBJECT: LIGO/Virgo/KAGRA S250704ab: Coverage and upper limits from MAXI/GSC observations
DATE:    25/07/04 06:49:21 GMT
FROM:    Yuta Kawakubo at Aoyama Gakuin University <kawakubo@phys.aoyama.ac.jp>

H. Negoro, M. Nakajima, K. Takagi (Nihon U.),
N. Kawai, T. Mihara, (RIKEN),
S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU)
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S250704ab at 2025-07-04 04:30:48.040 UTC (GCN #40935).

At the trigger time of S250704ab, the high-voltage of MAXI/GSC was on,
but the FOV was out of the 90% credible region of the bayestar skymap.
The first one-orbit (92 min) scan observation with GSC after the event covered 86%
of the 90% credible region of the bayestar skymap from 05:48:11 to 06:01:46 UTC (T0+4643 to T0+5458 sec).

No significant new source was found in the region in the one-orbit scan observation.
A typical 1-sigma averaged upper limit obtained in one scan observation
is 20 mCrab at 2-20 keV.

If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.

TITLE:   GCN CIRCULAR
NUMBER:  40955
SUBJECT: LIGO/Virgo/KAGRA S250704ab: Upper limits from Fermi-GBM Observations
DATE:    25/07/04 14:59:46 GMT
FROM:    Cuán de Barra at UCD <cuan.debarra@ucdconnect.ie>

C. de Barra (UCD) reports on behalf of the Fermi-GBM Team:

For S250704ab (GCN 40935) and using the initial bayestar skymap, Fermi-GBM was observing 100.0% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S250704ab. An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates.

Part of the LVK localization region is behind the Earth for Fermi, located at an RA=355.2, Dec=-25.3 with a radius of 68.0 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395, we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2):

Timescale  Soft   Normal   Hard 
------------------------------------
0.128 s: 3.2  4.7  8.2
1.024 s: 0.97 1.3  2.4
8.192 s: 0.34 0.39 0.68

Assuming the median luminosity distance of 536 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s):

Timescale  Soft     Normal   Hard
------------------------------------
0.128s: 0.17 0.23 0.66
1.024s: 0.05 0.06 0.19
8.192s: 0.02 0.02 0.05

TITLE:   GCN CIRCULAR
NUMBER:  40959
SUBJECT: LIGO/Virgo/KAGRA S250704ab: NED Galaxies in the 3-Initial Localization Volume
DATE:    25/07/04 20:47:24 GMT
FROM:    David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>

David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)

On behalf of the NASA/IPAC Extragalactic Database (NED) Team.

We spatially cross-matched the LVK S250704ab-3-Initial sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 1118 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:

Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250704ab/3                       
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250704ab/3/20

The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.

|                  objname|            ra|           dec|objtype|    DistMpc|DistMpc_unc|       m_NUV|   m_NUV_unc|        m_Ks|    m_Ks_unc|        m_W1|    m_W1_unc|    P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|
|WISEA J053258.59+491405.3|      83.24413|      49.23481|      G|     523.55|       null|        null|        null|      13.311|       0.109|       9.424|       0.006|7.71e-07|  9.97e-09|
|WISEA J053516.57+442623.7|      83.81906|      44.43992|      G|     509.25|       null|        null|        null|      13.033|       0.125|      10.237|       0.006|1.66e-06|  9.52e-09|
|WISEA J053539.37+465222.0|      83.91405|      46.87278|      G|     461.85|       null|        null|        null|      13.055|       0.145|      11.308|       0.009|4.66e-06|  8.19e-09|
|WISEA J053923.04+451743.3|      84.84603|      45.29538|      G|     539.56|       null|        null|        null|      12.453|       0.119|      11.620|       0.009|3.77e-06|  6.76e-09|
|WISEA J054250.35+455634.2|      85.70983|      45.94286|      G|     483.57|       null|        null|        null|      12.898|       0.119|      10.988|       0.007|2.12e-06|  5.57e-09|
|WISEA J053208.93+475055.0|      83.03725|      47.84862|      G|     557.44|       null|        null|        null|      13.767|       0.225|      11.121|       0.007|1.67e-06|  5.11e-09|
|WISEA J053723.16+460044.4|      84.34651|      46.01236|      G|     407.49|       null|        null|        null|      13.495|       0.191|      11.529|       0.007|4.32e-06|  4.87e-09|
|WISEA J053847.35+462114.1|      84.69732|      46.35394|      G|     470.87|       null|        null|        null|      12.551|       0.100|      12.067|       0.011|4.71e-06|  4.28e-09|
|WISEA J054001.86+462259.3|      85.00789|      46.38354|      G|     428.04|       null|        null|        null|      12.805|       0.107|      11.667|       0.008|3.64e-06|  3.93e-09|
|WISEA J053634.97+455156.1|      84.14573|      45.86561|      G|     530.56|       null|        null|        null|      13.218|       0.133|      12.261|       0.014|4.02e-06|  3.90e-09|
|WISEA J053325.56+455947.7|      83.35651|      45.99659|      G|     419.32|       null|        null|        null|      13.605|       0.224|      11.232|       0.006|2.42e-06|  3.76e-09|
|WISEA J053334.13+470433.8|      83.39222|      47.07607|      G|     371.60|       null|      20.842|       0.261|      13.005|       0.120|      11.052|       0.009|2.57e-06|  3.71e-09|
|              2MFGC 04519|      83.31378|      47.69047|      G|     438.26|       null|        null|        null|      11.800|       0.104|      11.609|       0.008|3.01e-06|  3.61e-09|
|WISEA J053715.47+424503.7|      84.31447|      42.75103|      G|     404.22|       null|        null|        null|      12.767|       0.107|       9.881|       0.006|6.85e-07|  3.45e-09|
|WISEA J053425.20+462348.5|      83.60502|      46.39682|      G|     552.92|       null|        null|        null|      13.309|       0.150|      12.061|       0.011|2.73e-06|  3.45e-09|
|WISEA J054541.19+423211.5|      86.42163|      42.53653|      G|     510.30|       null|        null|        null|      13.697|       0.200|      10.637|       0.006|8.61e-07|  3.43e-09|
|WISEA J053311.93+444510.8|      83.29972|      44.75302|      G|     386.56|       null|        null|        null|      12.995|       0.132|      10.209|       0.006|9.40e-07|  3.19e-09|
|WISEA J053625.76+453611.0|      84.10735|      45.60307|      G|     500.51|       null|        null|        null|      13.030|       0.166|      12.439|       0.024|4.07e-06|  2.98e-09|
|WISEA J054201.60+460028.5|      85.50669|      46.00793|      G|     563.41|       null|        null|        null|      12.939|       0.138|      11.887|       0.008|1.83e-06|  2.81e-09|
|WISEA J053805.55+471905.7|      84.52316|      47.31826|      G|     537.68|       null|        null|        null|      13.255|       0.144|      12.212|       0.010|2.69e-06|  2.79e-09|

Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250704ab sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). 

TITLE:   GCN CIRCULAR
NUMBER:  41033
SUBJECT: LIGO/Virgo/KAGRA S250704ab: Updated Sky localization and EM Bright Classification
DATE:    25/07/08 15:59:56 GMT
FROM:    lucy.thomas@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250704ab (GCN Circular 40935). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250704ab

Based on posterior support from parameter estimation [1], under the assumption that the candidate S250704ab is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 57 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(05h40m, +44d43m, 7.74d, 2.35d, 77.56d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 531 +/- 101 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe

TITLE:   GCN CIRCULAR
NUMBER:  41043
SUBJECT: LIGO/Virgo/KAGRA S250704ab: NED Galaxies in the 4-Update Localization Volume
DATE:    25/07/08 23:42:55 GMT
FROM:    David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>

David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)

On behalf of the NASA/IPAC Extragalactic Database (NED) Team.

We spatially cross-matched the LVK S250704ab-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 611 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:

Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250704ab/4                       
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250704ab/4/20

The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.

|                  objname|            ra|           dec|objtype|    DistMpc|DistMpc_unc|  m_NUV|m_NUV_unc|        m_Ks|    m_Ks_unc|        m_W1|    m_W1_unc|    P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|-------|---------|------------|------------|------------|------------|--------|----------|
|WISEA J053516.57+442623.7|      83.81906|      44.43992|      G|     509.25|       null|   null|     null|      13.033|       0.125|      10.237|       0.006|1.71e-06|  1.82e-08|
|WISEA J054250.35+455634.2|      85.70983|      45.94286|      G|     483.57|       null|   null|     null|      12.898|       0.119|      10.988|       0.007|3.12e-06|  1.51e-08|
|WISEA J053923.04+451743.3|      84.84603|      45.29538|      G|     605.62|       null|   null|     null|      12.453|       0.119|      11.620|       0.009|3.61e-06|  1.51e-08|
|WISEA J053539.37+465222.0|      83.91405|      46.87278|      G|     461.85|       null|   null|     null|      13.055|       0.145|      11.308|       0.009|4.47e-06|  1.46e-08|
|WISEA J054541.19+423211.5|      86.42163|      42.53653|      G|     510.30|       null|   null|     null|      13.697|       0.200|      10.637|       0.006|1.80e-06|  1.33e-08|
|WISEA J054353.77+433318.3|      85.97407|      43.55511|      G|     578.18|       null|   null|     null|      13.998|       0.142|      11.554|       0.010|2.53e-06|  1.03e-08|
|WISEA J053847.35+462114.1|      84.69732|      46.35394|      G|     470.87|       null|   null|     null|      12.551|       0.100|      12.067|       0.011|5.61e-06|  9.44e-09|
|WISEA J053634.97+455156.1|      84.14573|      45.86561|      G|     530.56|       null|   null|     null|      13.218|       0.133|      12.261|       0.014|4.81e-06|  8.65e-09|
|WISEA J053258.59+491405.3|      83.24413|      49.23481|      G|     523.55|       null|   null|     null|      13.311|       0.109|       9.424|       0.006|3.47e-07|  8.31e-09|
|WISEA J053723.16+460044.4|      84.34651|      46.01236|      G|     407.49|       null|   null|     null|      13.495|       0.191|      11.529|       0.007|3.82e-06|  7.98e-09|
|WISEA J054001.86+462259.3|      85.00789|      46.38354|      G|     428.04|       null|   null|     null|      12.805|       0.107|      11.667|       0.008|3.94e-06|  7.89e-09|
|WISEA J054201.60+460028.5|      85.50669|      46.00793|      G|     563.41|       null|   null|     null|      12.939|       0.138|      11.887|       0.008|2.69e-06|  7.68e-09|
|WISEA J053208.93+475055.0|      83.03725|      47.84862|      G|     557.44|       null|   null|     null|      13.767|       0.225|      11.121|       0.007|1.33e-06|  7.56e-09|
|WISEA J053425.20+462348.5|      83.60502|      46.39682|      G|     552.92|       null|   null|     null|      13.309|       0.150|      12.061|       0.011|2.86e-06|  6.71e-09|
|WISEA J054301.87+435708.0|      85.75780|      43.95222|      G|     559.29|       null|   null|     null|      13.811|       0.207|      12.385|       0.016|3.59e-06|  6.34e-09|
|WISEA J053625.76+453611.0|      84.10735|      45.60307|      G|     500.51|       null|   null|     null|      13.030|       0.166|      12.439|       0.024|4.61e-06|  6.27e-09|
|WISEA J054328.21+450858.0|      85.86756|      45.14945|      G|     449.66|       null|   null|     null|      13.406|       0.172|      11.996|       0.011|3.70e-06|  6.12e-09|
|WISEA J053715.47+424503.7|      84.31447|      42.75103|      G|     404.22|       null|   null|     null|      12.767|       0.107|       9.881|       0.006|6.42e-07|  6.01e-09|
|WISEA J053606.08+462143.1|      84.02537|      46.36199|      G|     573.70|       null|   null|     null|      13.989|       0.253|      12.477|       0.020|3.44e-06|  5.93e-09|
|WISEA J054047.69+451204.5|      85.19873|      45.20127|      G|     455.94|       null|   null|     null|      13.402|       0.074|      12.608|       0.024|6.10e-06|  5.84e-09|

Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250704ab sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). 

TITLE:   GCN CIRCULAR
NUMBER:  40884
SUBJECT: LIGO/Virgo/KAGRA S250702n: Identification of a GW compact binary merger candidate
DATE:    25/07/02 14:18:26 GMT
FROM:    Lorenzo Lunghini at University of Napoli  <lorenzo.lunghini@na.infn.it>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250702n during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2025-07-02 13:32:00.382 UTC (GPS time: 1435498338.382). The candidate was found by the cWB BBH [1], GstLAL [2], MBTA [3], and PyCBC Live [4] analysis pipelines.

S250702n is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.7e-08 Hz, or about one in 1 year, 9 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250702n

The classification of the GW signal, in order of descending probability, is BBH (99%), Terrestrial (1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 33 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 2243 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(04h30m, -39d49m, 37.85d, 19.28d, 56.35d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2096 +/- 697 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [5] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [6] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41032
SUBJECT: LIGO/Virgo/KAGRA S250702n: Updated Sky localization
DATE:    25/07/08 15:46:53 GMT
FROM:    lucy.thomas@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250702n (GCN Circular 40884). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250702n

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 1926 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(04h22m, -40d00m, 33.59d, 18.62d, 58.18d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2180 +/- 913 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  40880
SUBJECT: LIGO/Virgo/KAGRA S250701bq: Identification of a GW compact binary merger candidate
DATE:    25/07/02 00:04:55 GMT
FROM:    이형원 (Hyung) at Inje University <ijhwlee@gmail.com>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250701bq during real-time processing of data from LIGO Livingston Observatory (L1) at 2025-07-01 23:17:46.927 UTC (GPS time: 1435447084.927). The candidate was found by the GstLAL [1] analysis pipeline.

S250701bq is an event of interest because its false alarm rate, as estimated by the online analysis, is 4.3e-17 Hz, or about one in 1e9 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250701bq

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (11.0, 22.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 23 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 24219 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 556 +/- 171 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41031
SUBJECT: LIGO/Virgo/KAGRA S250701bq: Updated Sky localization
DATE:    25/07/08 15:39:52 GMT
FROM:    lucy.thomas@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250701bq (GCN Circular 40880). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250701bq

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 25010 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 480 +/- 118 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  40879
SUBJECT: LIGO/Virgo/KAGRA S250701bp: Identification of a GW compact binary merger candidate
DATE:    25/07/01 21:05:18 GMT
FROM:    mayara.pacheco@inpe.br

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250701bp during real-time processing of data from LIGO Livingston Observatory (L1) at 2025-07-01 20:34:40.856 UTC (GPS time: 1435437298.856). The candidate was found by the GstLAL [1] analysis pipeline.

S250701bp is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.1e-17 Hz, or about one in 1e9 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250701bp

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 24220 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 958 +/- 300 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  41029
SUBJECT: LIGO/Virgo/KAGRA S250701bp: Updated Sky localization
DATE:    25/07/08 15:05:19 GMT
FROM:    lucy.thomas@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250701bp (GCN Circular 40879). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250701bp

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 26503 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 939 +/- 300 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  40869
SUBJECT: LIGO/Virgo/KAGRA S250629bs: Identification of a GW compact binary merger candidate
DATE:    25/06/29 13:05:07 GMT
FROM:    rodrigo.cuzinatto@unifal-mg.edu.br

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250629bs during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2025-06-29 12:13:04.377 UTC (GPS time: 1435234402.377). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], and MBTA [4] analysis pipelines.

S250629bs is an event of interest because its false alarm rate, as estimated by the online analysis, is 4.3e-08 Hz, or about one in 8 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250629bs

The classification of the GW signal, in order of descending probability, is BBH (94%), Terrestrial (6%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 23 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [6], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 3008 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1811 +/- 561 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [6] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  40873
SUBJECT: LIGO/Virgo/KAGRA S250629bs: Updated Sky localization
DATE:    25/06/29 16:37:41 GMT
FROM:    John Veitch at U of Glasgow <john.veitch@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250629bs (GCN Circular 40869). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250629bs

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 2998 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4847 +/- 1746 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41058
SUBJECT: LIGO/Virgo/KAGRA S250629bs: Upper limits from Swift/BAT-GUANO
DATE:    25/07/10 14:47:59 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 86.28% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250629bs/files/Bilby.multiorder.fits)) at merger time. A fraction 64.89% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |3.65  |3.14  |2.87  |3.39  
|1.024        |1.87  |1.61  |1.47  |1.74  
|4.096        |1.02  |0.88  |0.80  |0.95  
|16.384       |0.65  |0.56  |0.51  |0.60  


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://zenodo.org/records/15843754

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

TITLE:   GCN CIRCULAR
NUMBER:  40867
SUBJECT: LIGO/Virgo/KAGRA S250629ae: Identification of a GW compact binary merger candidate
DATE:    25/06/29 06:56:02 GMT
FROM:    林峻哲Lin, Chun-Che <lupin@phys.ncku.edu.tw>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250629ae during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-06-29 05:50:36.925 UTC (GPS time: 1435211454.925). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], and PyCBC Live [5] analysis pipelines.

S250629ae is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-08 Hz, or about one in 11 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250629ae

The classification of the GW signal, in order of descending probability, is BBH (98%), Terrestrial (2%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 40 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 3184 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4550 +/- 1295 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  40870
SUBJECT: LIGO/Virgo/KAGRA S250629ae: Updated Sky localization
DATE:    25/06/29 13:31:48 GMT
FROM:    Aditya Vijaykumar <aditya.vijaykumar@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250629ae (GCN Circular 40867). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250629ae

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 2834 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 7075 +/- 2116 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  41057
SUBJECT: LIGO/Virgo/KAGRA S250629ae: Upper limits from Swift/BAT-GUANO
DATE:    25/07/10 14:43:39 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 50.9% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250629ae/files/Bilby.multiorder.fits)) at merger time. A fraction 42.48% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |2.96  |2.54  |2.33  |2.73  
|1.024        |1.51  |1.29  |1.19  |1.39  
|4.096        |0.81  |0.69  |0.63  |0.74  
|16.384       |0.49  |0.42  |0.39  |0.45  


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://zenodo.org/records/15843728

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

TITLE:   GCN CIRCULAR
NUMBER:  40863
SUBJECT: LIGO/Virgo/KAGRA S250628am: Identification of a GW compact binary merger candidate
DATE:    25/06/28 19:08:45 GMT
FROM:    F. Llamas <francisco.llamas@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250628am during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-06-28 18:23:19.785 UTC (GPS time: 1435170217.785). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S250628am is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.7e-14 Hz, or about one in 1e6 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250628am

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 32 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 427 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1112 +/- 273 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  40866
SUBJECT: LIGO/Virgo/KAGRA S250628am: Updated Sky localization
DATE:    25/06/28 21:09:56 GMT
FROM:    Aditya Vijaykumar <aditya.vijaykumar@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250628am (GCN Circular 40863). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250628am

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 287 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 902 +/- 223 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  40868
SUBJECT:  LIGO/Virgo/KAGRA S250628am: Identification of a GW compact binary merger candidate (duplicate submission)
DATE:    25/06/29 12:55:27 GMT
FROM:    rodrigo.cuzinatto@unifal-mg.edu.br

This is a duplicate of Circular [40863](https://gcn.nasa.gov/circulars/40863).

TITLE:   GCN CIRCULAR
NUMBER:  39953
SUBJECT: LIGO/Virgo/KAGRA S250331o: Identification of a GW compact binary merger candidate
DATE:    25/03/31 02:06:54 GMT
FROM:    Gyoik Kim at Yonsei University <gyoik_kim@yonsei.ac.kr>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250331o during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-03-31 01:34:48.438 UTC (GPS time: 1427420106.438). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S250331o is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250331o

After parameter estimation by RapidPE-RIFT [6], the classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN notice about 23 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 27 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(02h07m, +31d24m, 4.57d, 1.87d, 116.86d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 582 +/- 120 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [5] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [6] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002 (2015) doi:10.1103/PhysRevD.92.023002
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  39975
SUBJECT: LIGO/Virgo/KAGRA S250331o: NED Galaxies in the 3-Initial Localization Volume
DATE:    25/03/31 15:48:32 GMT
FROM:    David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>

David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)

On behalf of the NASA/IPAC Extragalactic Database (NED) Team.

We spatially cross-matched the LVK S250331o-3-Initial sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 582 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:

Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250331o/3                       
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250331o/3/20

The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.

|                  objname|            ra|           dec|objtype|    DistMpc|DistMpc_unc|       m_NUV|   m_NUV_unc|        m_Ks|    m_Ks_unc|        m_W1|    m_W1_unc|    P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|
|WISEA J020352.96+300418.6|      30.97068|      30.07186|      G|     450.96|       null|        null|        null|      13.417|       0.135|      10.147|       0.006|2.21e-06|  3.99e-08|
|WISEA J020524.84+300356.4|      31.35354|      30.06568|      G|     653.76|       null|        null|        null|      13.537|       0.143|      12.061|       0.009|1.69e-06|  1.10e-08|
|             3C 059 NED01|      31.75919|      29.51278|      G|     510.64|       2.89|      18.553|       0.047|      12.531|       0.068|      11.452|       0.006|1.47e-06|  1.02e-08|
|WISEA J020815.86+330236.7|      32.06612|      33.04354|      G|     528.82|       null|        null|        null|      12.409|       0.098|      12.307|       0.010|1.99e-06|  6.80e-09|
|WISEA J020728.78+301402.8|      31.86992|      30.23411|      G|     488.46|       null|      22.228|       0.345|      12.899|       0.118|      12.205|       0.008|2.12e-06|  6.76e-09|
|WISEA J021035.33+303608.5|      32.64721|      30.60238|      G|     569.79|       null|        null|        null|      12.895|       0.117|      12.315|       0.013|1.64e-06|  6.45e-09|
|WISEA J020735.81+301139.9|      31.89924|      30.19442|      G|     697.62|       null|        null|        null|      13.258|       0.120|      12.206|       0.008|9.86e-07|  6.42e-09|
|WISEA J020447.91+305422.6|      31.19964|      30.90630|      G|     421.78|       null|        null|        null|      12.281|       0.097|      11.835|       0.007|1.92e-06|  6.41e-09|
|WISEA J021036.53+303612.4|      32.65223|      30.60347|      G|     607.46|       null|        null|        null|      13.762|       0.178|      12.435|       0.008|1.55e-06|  6.19e-09|
|WISEA J020031.78+285022.0|      30.13243|      28.83945|      G|     548.45|       null|        null|        null|      13.274|       0.142|      12.152|       0.008|1.32e-06|  5.57e-09|
|WISEA J020247.16+295941.4|      30.69650|      29.99484|      G|     557.31|       null|      20.629|       0.118|      13.505|       0.133|      12.951|       0.014|2.62e-06|  5.47e-09|
|WISEA J020311.55+300127.5|      30.79816|      30.02432|      G|     512.31|       0.08|        null|        null|      13.071|       0.101|      12.808|       0.010|2.61e-06|  5.24e-09|
|WISEA J021613.00+340524.3|      34.05421|      34.09010|      G|     677.00|       null|        null|        null|      13.485|       0.160|      11.789|       0.007|5.73e-07|  5.16e-09|
|WISEA J020959.78+325524.0|      32.49910|      32.92334|      G|     589.92|       0.10|      21.761|       0.336|      13.738|       0.179|      13.027|       0.012|2.33e-06|  5.10e-09|
|WISEA J020619.97+294730.7|      31.58324|      29.79188|      G|     507.55|       0.07|        null|        null|      12.533|       0.104|      12.599|       0.020|2.12e-06|  5.09e-09|
|WISEA J020633.75+303828.7|      31.64066|      30.64131|      G|     429.37|       null|        null|        null|      12.181|       0.101|      12.041|       0.008|1.73e-06|  4.95e-09|
|                VI Zw 139|      30.40470|      30.82735|      G|     509.90|       0.06|        null|        null|      13.148|       0.120|      12.551|       0.009|1.91e-06|  4.80e-09|
|WISEA J020659.62+303130.8|      31.74842|      30.52523|      G|     462.52|       null|      21.179|       0.192|      13.405|       0.144|      12.477|       0.009|2.02e-06|  4.49e-09|
|WISEA J020919.83+314709.5|      32.33265|      31.78598|      G|     656.06|       0.12|        null|        null|      14.209|       0.268|      13.409|       0.016|2.34e-06|  4.46e-09|
|WISEA J020626.94+302913.7|      31.61227|      30.48715|      G|     672.91|       0.08|        null|        null|      13.829|       0.091|      13.152|       0.013|1.69e-06|  4.27e-09|

Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250331o sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). 

TITLE:   GCN CIRCULAR
NUMBER:  39986
SUBJECT: LIGO/Virgo/KAGRA S250331o: Updated Sky localization
DATE:    25/03/31 19:50:46 GMT
FROM:    Michael J. Williams at University of Portsmouth <michael.williams@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250331o (GCN Circular 39953). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250331o

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 18 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(02h07m, +31d03m, 3.98d, 1.46d, 114.57d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 657 +/- 91 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  40109
SUBJECT: LIGO/Virgo/KAGRA S250331o: Upper limits from Swift/BAT-GUANO
DATE:    25/04/07 19:04:57 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 100.0% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250331o/files/Bilby.multiorder.fits)) at merger time. A fraction 96.02% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |3.32  |3.49  |3.33  |3.61  
|1.024        |1.69  |1.78  |1.70  |1.84  
|4.096        |0.91  |0.95  |0.91  |0.99  
|16.384       |0.55  |0.58  |0.56  |0.60  


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://zenodo.org/records/15169889

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

TITLE:   GCN CIRCULAR
NUMBER:  39898
SUBJECT: LIGO/Virgo/KAGRA S250328ae: Identification of a GW compact binary merger candidate
DATE:    25/03/28 06:27:04 GMT
FROM:    Sayantan Ghosh at Indian Institute of Technology Bombay <stanghosh@iitb.ac.in>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250328ae during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-03-28 05:40:27.419 UTC (GPS time: 1427175645.419). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], MLy [5], PyCBC Live [6], and SPIIR [7] analysis pipelines.

S250328ae is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250328ae

After parameter estimation by RapidPE-RIFT [8], the classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [9] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [9] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 7%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [10], distributed via GCN notice about 32 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [10], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 22 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(09h39m, +10d34m, 7.42d, 0.95d, 115.70d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 492 +/- 91 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
 [5] Skliris et al. (2020) arXiv:2009.14611
 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [8] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002 (2015) doi:10.1103/PhysRevD.92.023002
 [9] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [10] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  39900
SUBJECT: LIGO/Virgo/KAGRA S250328ae: Updated Sky localization
DATE:    25/03/28 08:07:05 GMT
FROM:    Aaron Zimmerman at U. of Texas at Austin <aaron.zimmerman@utexas.edu>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250328ae (GCN Circular 39898). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250328ae

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 15 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(09h39m, +10d53m, 4.94d, 0.99d, 116.27d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 511 +/- 82 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  39908
SUBJECT: LIGO/Virgo/KAGRA S250328ae: NED Galaxies in the 4-Update Localization Volume
DATE:    25/03/28 15:33:03 GMT
FROM:    David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>

David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)

On behalf of the NASA/IPAC Extragalactic Database (NED) Team.

We spatially cross-matched the LVK S250328ae-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 839 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:

Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250328ae/4                       
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250328ae/4/20

The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.

|                  objname|            ra|           dec|objtype|    DistMpc|DistMpc_unc|       m_NUV|   m_NUV_unc|        m_Ks|    m_Ks_unc|        m_W1|    m_W1_unc|    P_3D|P_3D_LumW1|
|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|
|WISEA J094227.03+110047.4|     145.61266|      11.01318|      G|     612.35|       null|        null|        null|      12.661|       0.096|      10.577|       0.006|2.63e-06|  5.47e-08|
|WISEA J093435.61+082700.3|     143.64838|       8.45010|      G|     507.10|       0.11|        null|        null|      13.186|       0.203|      10.854|       0.006|3.85e-06|  4.28e-08|
|WISEA J093458.83+094902.6|     143.74516|       9.81741|      G|     564.56|       0.09|        null|        null|      14.250|       0.157|      10.944|       0.007|2.12e-06|  2.69e-08|
|WISEA J093555.60+093752.8|     143.98169|       9.63134|      G|     589.67|       0.08|        null|        null|      13.465|       0.202|      11.501|       0.007|2.87e-06|  2.37e-08|
|WISEA J093938.05+110947.9|     144.90857|      11.16332|      G|     482.07|       0.08|        null|        null|      13.597|       0.141|      12.972|       0.014|1.58e-05|  2.25e-08|
|WISEA J094028.13+111733.4|     145.11722|      11.29263|      G|     556.24|       0.11|        null|        null|      13.110|       0.111|      12.941|       0.015|9.93e-06|  1.94e-08|
|WISEA J094035.77+103104.6|     145.14905|      10.51795|      G|     495.66|       0.07|        null|        null|      13.045|       0.080|      12.298|       0.012|6.77e-06|  1.90e-08|
|WISEA J093657.39+085125.0|     144.23916|       8.85696|      G|     557.28|       0.06|        null|        null|        null|        null|      11.769|       0.007|3.22e-06|  1.86e-08|
|WISEA J094109.87+105038.6|     145.29116|      10.84407|      G|     439.67|       0.07|        null|        null|      12.109|       0.080|      12.120|       0.012|6.55e-06|  1.70e-08|
|WISEA J094121.23+112617.1|     145.33849|      11.43811|      G|     564.36|       0.09|        null|        null|      13.311|       0.123|      13.095|       0.017|9.76e-06|  1.70e-08|
|WISEA J094025.05+111347.9|     145.10441|      11.23000|      G|     557.91|       0.09|        null|        null|      13.243|       0.108|      13.078|       0.020|9.62e-06|  1.66e-08|
|WISEA J093619.16+091944.6|     144.07983|       9.32906|      G|     588.58|       0.11|        null|        null|      13.919|       0.167|      12.015|       0.007|3.16e-06|  1.62e-08|
|WISEA J093619.13+091936.4|     144.07971|       9.32680|      G|     591.85|       0.08|        null|        null|      12.365|       0.106|      12.060|       0.010|3.00e-06|  1.49e-08|
|WISEA J093747.49+101024.7|     144.44789|      10.17354|      G|     432.26|       0.05|        null|        null|      12.881|       0.108|      12.438|       0.013|7.81e-06|  1.46e-08|
|WISEA J093417.20+081006.5|     143.57167|       8.16848|      G|     347.36|       0.08|      21.156|       0.071|      13.681|       0.191|      10.002|       0.006|1.22e-06|  1.39e-08|
|WISEA J094125.76+110548.3|     145.35737|      11.09678|      G|     557.02|       0.07|        null|        null|      13.696|       0.126|      13.350|       0.021|9.58e-06|  1.28e-08|
|WISEA J094051.38+102028.4|     145.21409|      10.34125|      G|     444.19|       0.03|        null|        null|      15.275|       0.145|      12.091|       0.007|4.61e-06|  1.25e-08|
|WISEA J094106.66+105024.2|     145.27777|      10.84008|      G|     487.71|       null|        null|        null|      14.219|       0.109|      13.198|       0.021|1.00e-05|  1.18e-08|
|WISEA J093537.73+091115.6|     143.90721|       9.18768|      G|     552.71|       0.11|        null|        null|      13.648|       0.222|      12.528|       0.013|4.10e-06|  1.16e-08|
|WISEA J093620.06+092335.0|     144.08362|       9.39307|      G|     582.84|       0.07|      20.401|       0.231|      13.508|       0.189|      12.534|       0.014|3.60e-06|  1.12e-08|

Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250328ae sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). 

TITLE:   GCN CIRCULAR
NUMBER:  39934
SUBJECT: LIGO/Virgo/KAGRA S250328ae: DECam DESGW Candidates (Epoch 1)
DATE:    25/03/29 22:30:24 GMT
FROM:    Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>

S. MacBride (UZH), I. McMahon (UZH), S. Kaur (University of Michigan/UZH), M. Soares-Santos (UZH), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 01:43 UTC, March 29th, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250328ae (GCN 39898). We observed eight fields centered on the following ICRS coordinates:
(145.286221, 10.615360)
(145.996500, 12.108130)
(143.786221, 10.715360)
(144.496500, 12.208130)
(144.069167, 9.115670)
(143.344621, 7.609550)
(145.900667, 13.593470)
(142.620092, 6.103430)
These pointings cover the 90% localization region of candidate gravitational-wave event S250328ae.

All fields were observed in DECam r, i, and z filters with 90-second exposures. The limiting magnitude achieved is ~22.7 in r-band, ~22.1 in i-band, and ~21.4 in z-band. 

We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were selected by requiring at least two high signal to noise detections. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.

After candidate selection, we report the 20 high confidence candidates listed below. After vetting and identification, 8 candidates are classified as nuclear candidates (likely active galactic nuclei), 8 candidates as possible supernovae, and 3 other candidates do not fall in either category. 1 additional candidate (2292763) has been identified as a known AGN WISEA J093710.05+082057.2. We have numerous other transient candidates which require further observation to confidently classify. Further observations are ongoing, and we encourage followup of the 20 candidates identified herein. 

| TYPE | ID | ATNAME | RA | DEC | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |
| -------- | ------- | --------- | ---------- | ---------- | ----- | ---- | ----- | ---- | ----- | ---- |
| SN_LIKE | 2290892 | AT2025gei | 143.636743 | 9.980209 | N/A | N/A | 20.59 | 0.03 | N/A | N/A |
| OTHER | 2292987 | AT2025gej | 143.183373 | 6.893136 | 21.37 | 0.04 | 21.45 | 0.08 | N/A | N/A |
| AGN_LIKE | 2290036 | AT2025gek | 145.199481 | 10.828527 | 20.77 | 0.04 | 20.62 | 0.03 | 20.69 | 0.1 |
| SN_LIKE | 2290467 | AT2020woa | 144.388798 | 10.255536 | 19.87 | 0.02 | N/A | N/A | 20.1 | 0.04 |
| SN_LIKE | 2290524 | AT2025gel | 144.524917 | 10.199936 | N/A | N/A | N/A | N/A | 20.47 | 0.06 |
| OTHER | 2290001 | AT2025avp | 144.806008 | 10.632223 | 20.36 | 0.02 | 20.19 | 0.02 | 20.22 | 0.06 |
| SN_LIKE | 2291473 | AT2025gem | 144.706509 | 11.560317 | 20.92 | 0.03 | N/A | N/A | 21.22 | 0.1 |
| AGN_LIKE | 2293190 | AT2025cvb | 144.191297 | 10.828462 | 20.86 | 0.03 | 20.87 | 0.03 | N/A | N/A |
| AGN_LIKE | 2290623 | AT2025gen | 144.112464 | 11.641817 | 21.4 | 0.04 | 21.3 | 0.08 | 21.37 | 0.12 |
| SN_LIKE | 2292782 | AT2025geo | 144.928578 | 10.349243 | 20.92 | 0.04 | 21.07 | 0.04 | N/A | N/A |
| SN_LIKE | 2291779 | AT2025gej | 143.182187 | 6.894174 | N/A | N/A | N/A | N/A | 20.76 | 0.06 |
| OTHER | 2290762 | AT2025gep | 144.259773 | 11.511316 | 21.29 | 0.04 | N/A | N/A | N/A | N/A |
| SN_LIKE | 2290334 | AT2025geq | 145.235616 | 12.469694 | 20.22 | 0.02 | 20.57 | 0.03 | N/A | N/A |
| AGN_LIKE | 2290970 | AT2025ger | 142.543835 | 6.090899 | 21.71 | 0.05 | 21.43 | 0.06 | 21.28 | 0.09 |
| AGN_LIKE | 2292040 | AT2025ges | 144.438216 | 9.885617 | 21.72 | 0.07 | 21.75 | 0.11 | 21.93 | 0.17 |
| SN_LIKE | 2292645 | AT2025get | 144.053408 | 11.422277 | N/A | N/A | 22.67 | 0.21 | N/A | N/A |
| AGN_LIKE | 2293517 | AT2025geu | 145.956024 | 12.381673 | 21.73 | 0.08 | 21.65 | 0.07 | N/A | N/A |
| AGN_LIKE | 2291833 | AT2025gev | 143.241035 | 8.651865 | 22.16 | 0.09 | 21.99 | 0.13 | 22.22 | 0.22 |
| AGN_LIKE | 2294074 | AT2025gew | 144.291829 | 8.349302 | 22.23 | 0.1 | 21.85 | 0.11 | N/A | N/A |
| AGN | 2292763 | AT2025gex | 146.071329 | 11.839902 | 21.89 | 0.11 | 21.92 | 0.12 | 21.05 | 0.1 |

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.

TITLE:   GCN CIRCULAR
NUMBER:  39972
SUBJECT: LIGO/Virgo/KAGRA S250328ae: Swift XRT observations, 34 possible X-ray sources
DATE:    25/03/31 14:04:07 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S.B. Cenko (NASA/GSFC),
R.A.J. Eyles-Ferris (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G.
Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S. Campana (INAF-OAB), J.J. Delaunay (PSU), M. De
Pasquale (University of Messina), S. Dichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì
(INAF-IASFPA) , V. D’Elia (ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson
University), N. Klingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha
(NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien (U. Leicester),
M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato (INAF-OAB), B. Sbarufatti
(INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB), E. Troja (U Tor Vergata, INAF) report on
behalf of the Swift team:

Swift has carried out 97 observations of the LVK error region for the GW trigger S250328ae
convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9), using the 'BAYESTAR' GW
localisation map. As this is a 3D skymap, galaxy distances were taken into account in selecting
which ones to observe. The observations currently span from 3.0 ks to 38 ks after the LVK trigger,
and the XRT has covered 5.3 deg^2 on the sky (corrected for overlaps). This covers 68% of the
probability in the 'ligo-skymap-from-samples' skymap, and 72% after convolving with the 2MPZ galaxy
catalogue, as described by Evans et al. (2016, MNRAS,  462, 1591).

We have detected 34 X-ray sources. Each source is assigned a rank of 1-4 which describes how likely
it is to be related to the GW trigger, with 1 being the most likely and 4 being the least likely.
The ranks are described at https://www.swift.ac.uk/ranks.php.

We have found:

  * 0 sources of rank 1
  * 0 sources of rank 2
  * 34 sources of rank 3
  * 0 sources of rank 4


RANK 3 sources
==============

These are uncatalogued X-ray sources, however they are not brighter than previous upper limits, so
do not stand out as likely counterparts to the GW trigger.

| Source ID	 | RA		  | Dec 	   | Err90   | Detection Flag	|
|  S250328ae_X1  | 09h 37m 34.26s | +11d 00' 47.1" |	8.3" |		   POOR |
|  S250328ae_X2  | 09h 37m 40.59s | +11d 15' 09.3" |	5.4" |	     REASONABLE |
|  S250328ae_X3  | 09h 38m 01.16s | +11d 14' 02.6" |	6.5" |		   POOR |
|  S250328ae_X5  | 09h 38m 05.54s | +11d 24' 32.5" |	6.5" |		   POOR |
|  S250328ae_X6  | 09h 37m 57.28s | +11d 18' 05.4" |	6.5" |		   POOR |
|  S250328ae_X8  | 09h 38m 05.52s | +11d 19' 00.3" |	9.1" |		   POOR |
|  S250328ae_X9  | 09h 38m 34.55s | +11d 14' 28.5" |	6.4" |		   POOR |
| S250328ae_X10  | 09h 38m 37.34s | +11d 16' 12.5" |	9.9" |		   POOR |
| S250328ae_X11  | 09h 38m 13.89s | +11d 16' 52.4" |	9.5" |		   POOR |
| S250328ae_X12  | 09h 37m 35.17s | +10d 47' 58.5" |	6.6" |		   POOR |
| S250328ae_X13  | 09h 39m 29.53s | +11d 47' 43.2" |	6.0" |		   POOR |
| S250328ae_X14  | 09h 39m 57.31s | +12d 06' 58.4" |	8.7" |		   POOR |
| S250328ae_X15  | 09h 39m 31.81s | +12d 06' 34.6" |	7.9" |		   POOR |
| S250328ae_X16  | 09h 40m 10.28s | +11d 53' 17.1" |	6.7" |		   POOR |
| S250328ae_X17  | 09h 39m 48.52s | +11d 55' 48.6" |	7.4" |		   POOR |
| S250328ae_X18  | 09h 39m 45.99s | +11d 54' 42.0" |	7.5" |		   POOR |
| S250328ae_X19  | 09h 39m 39.79s | +11d 54' 32.3" |	6.6" |		   POOR |
| S250328ae_X20  | 09h 39m 23.21s | +11d 57' 42.7" |	6.6" |		   POOR |
| S250328ae_X21  | 09h 39m 41.64s | +11d 53' 24.9" |   13.1" |		   POOR |
| S250328ae_X22  | 09h 39m 45.64s | +11d 51' 32.9" |	9.1" |		   POOR |
| S250328ae_X23  | 09h 39m 28.69s | +11d 59' 35.2" |	8.2" |		   POOR |
| S250328ae_X24  | 09h 39m 51.34s | +11d 52' 24.6" |	6.2" |		   POOR |
| S250328ae_X25  | 09h 40m 08.46s | +11d 55' 52.5" |	8.9" |		   POOR |
| S250328ae_X26  | 09h 39m 51.33s | +11d 55' 00.9" |	6.6" |		   POOR |
| S250328ae_X27  | 09h 39m 34.55s | +12d 01' 50.9" |	7.2" |		   POOR |
| S250328ae_X28  | 09h 40m 17.23s | +11d 54' 47.3" |	9.7" |		   POOR |
| S250328ae_X29  | 09h 39m 31.19s | +11d 54' 35.1" |	8.4" |		   POOR |
| S250328ae_X30  | 09h 39m 39.91s | +12d 04' 29.3" |   15.4" |		   POOR |
| S250328ae_X31  | 09h 38m 40.15s | +11d 14' 00.3" |	5.7" |		   POOR |
| S250328ae_X32  | 09h 38m 17.68s | +11d 19' 25.6" |   13.5" |		   POOR |
| S250328ae_X33  | 09h 39m 18.34s | +11d 59' 40.6" |	8.9" |		   POOR |
| S250328ae_X34  | 09h 39m 52.80s | +11d 54' 16.5" |	8.0" |		   POOR |
| S250328ae_X35  | 09h 39m 34.91s | +12d 05' 56.6" |	9.0" |		   POOR |
| S250328ae_X36  | 09h 39m 29.20s | +12d 02' 51.1" |   10.7" |		   POOR |

For all flux conversions and comparisons with catalogues and upper limits from other missions, we
assumed a power-law spectrum with NH=3x10^20 cm^-2, and photon index (Gamma)=1.7, unless otherwise
stated.

The results of the XRT automated analysis, including details of the sources listed above, are
online at https://www.swift.ac.uk/LVC/S250328ae

This circular is an official product of the Swift XRT team.

TITLE:   GCN CIRCULAR
NUMBER:  39992
SUBJECT: LIGO/Virgo/KAGRA S250328ae: DECam DESGW Candidates (Epoch 2)
DATE:    25/04/01 01:35:09 GMT
FROM:    Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>

S. MacBride (UZH), I. McMahon (UZH), M. Soares-Santos (UZH), K. Herner (Fermilab), S. Kaur (University of Michigan/UZH), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 01:46 UTC, March 30th, the Dark Energy Camera (DECam) began the second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250328ae (GCN 39898). We observed eight fields centered on the following ICRS coordinates:

| RA | DEC |
| ---------- | ---------- |
| 145.286221 | 10.615360 |
| 145.996500 | 12.108130 |
| 143.786221 | 10.715360 |
| 144.496500 | 12.208130 |
| 144.069167 | 9.115670 |
| 143.344621 | 7.609550 |
| 145.900667 | 13.593470 |
| 142.620092 | 6.103430 |

These pointings are identical to the first epoch observations from March 29th, 2025 (GCN 39934).

All fields were observed in DECam r, i, and z filters with 90-second exposures. The limiting magnitude achieved is ~22.8 in r-band, ~22.5 in i-band, and ~21.9 in z-band. 

We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts, requiring an autoscan score of at least 0.7 on both nights of observations. We also match our candidates against the ALLWISE, Milliquas, Quaia, and LQAC-6 AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024, Souchay et al 2024) within the LVK localization volume.

After candidate selection, we report 25 high confidence candidates listed below. After vetting and identification, 15 candidates are classified as nuclear candidates (likely active galactic nuclei) and 10 candidates as possible supernovae. No candidates from the second epoch of observations match known AGNs within 1 arcsecond. 12 of the 20 candidates in the first epoch are listed here, while the candidates which do not appear did not have a confident detection in the second epoch. We also do not recover any of the X-ray source candidates reported by Swift XRT (GCN 39972) within their reported error bounds.

| TYPE | ID | ATNAME | RA | DEC | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |
| -------- | ------- | --------- | ---------- | ---------- | ----- | ---- | ----- | ---- | ----- | ---- |
| SN_LIKE | 2290036 | AT2025gek | 145.199481 | 10.828527 | 20.77 | 0.04 | 20.62 | 0.03 | 20.69 | 0.1 |
| SN_LIKE | 2290467 | AT2020woa | 144.388798 | 10.255536 | 19.87 | 0.02 | N/A  | N/A  | 20.1 | 0.04 |
| SN_LIKE | 2292782 | AT2025geo | 144.928578 | 10.349243 | 20.92 | 0.04 | 21.07 | 0.04 | N/A | N/A  |
| SN_LIKE | 2290001 | AT2025avp | 144.806008 | 10.632223 | 20.36 | 0.02 | 20.19 | 0.02 | 20.22 | 0.06 |
| SN_LIKE | 2290334 | AT2025geq | 145.235616 | 12.469694 | 20.22 | 0.02 | 20.57 | 0.03 | 21.11 | 0.11 |
| AGN_LIKE | 2290623 | AT2025gen | 144.112464 | 11.641817 | 21.4 | 0.04 | 21.3 | 0.08 | 21.37 | 0.12 |
| AGN_LIKE | 2291223 | N/A | 146.389563 | 13.131801 | 20.56 | 0.02 | 20.64 | 0.04 | 20.52 | 0.05 |
| SN_LIKE | 2291473 | AT2025gem | 144.706509 | 11.560317 | 20.92 | 0.03 | N/A  | N/A  | 21.22 | 0.1 |
| SN_LIKE | 2293190 | AT2025cvb | 144.191297 | 10.828462 | 20.86 | 0.03 | 20.87 | 0.03 | N/A | N/A  |
| SN_LIKE | 2290143 | AT2025ggv | 146.033906 | 11.088812 | 21.26 | 0.05 | 21.46 | 0.07 | 21.47 | 0.13 |
| AGN_LIKE | 2292003 | N/A | 144.007084 | 9.336975 | 21.18 | 0.03 | 21.16 | 0.05 | 21.6 | 0.11 |
| SN_LIKE | 2292854 | AT2025ggw | 142.67125 | 6.637349 | N/A  | N/A  | 20.75 | 0.04 | 20.72 | 0.06 |
| AGN_LIKE | 2290453 | N/A | 146.299999 | 13.629839 | 21.7 | 0.06 | 21.89 | 0.11 | 22.14 | 0.19 |
| AGN_LIKE | 2292040 | AT2025ges | 144.438216 | 9.885617 | 21.72 | 0.07 | 21.75 | 0.11 | 21.93 | 0.17 |
| AGN_LIKE | 2290059 | N/A | 144.679961 | 10.018211 | N/A  | N/A  | 20.26 | 0.03 | 21.11 | 0.12 |
| AGN_LIKE | 2290970 | AT2025ger | 142.543835 | 6.090899 | 21.71 | 0.05 | 21.43 | 0.06 | 21.28 | 0.09 |
| AGN_LIKE | 2294074 | AT2025gew | 144.291829 | 8.349302 | 22.23 | 0.1 | 21.85 | 0.11 | 22.28 | 0.15 |
| AGN_LIKE | 2292786 | N/A | 144.037664 | 10.810454 | 22.27 | 0.08 | 22.12 | 0.11 | 22.89 | 0.25 |
| AGN_LIKE | 2293517 | AT2025geu | 145.956024 | 12.381673 | 21.73 | 0.08 | 21.65 | 0.07 | 21.51 | 0.14 |
| AGN_LIKE | 2295862 | N/A | 145.361115 | 12.521385 | 22.44 | 0.11 | 23.33 | 0.36 | 23.47 | 0.46 |
| AGN_LIKE | 2290563 | N/A | 145.149074 | 11.756403 | 23.41 | 0.33 | 22.11 | 0.11 | 22.3 | 0.24 |
| AGN_LIKE | 2290298 | N/A | 144.167668 | 10.985136 | 21.59 | 0.05 | 21.99 | 0.12 | 21.1 | 0.1 |
| SN_LIKE | 2289995 | AT2025ggx | 144.57373 | 10.274445 | 22.62 | 0.19 | 22.31 | 0.14 | 21.7 | 0.22 |
| AGN_LIKE | 2291169 | N/A | 145.423743 | 11.834742 | 22.57 | 0.14 | 23.44 | 0.34 | 22.8 | 0.46 |
| AGN_LIKE | 2293519 | N/A | 143.751725 | 9.714206 | 22.75 | 0.15 | 22.23 | 0.14 | 21.63 | 0.12 |

We encourage followup of the 25 candidates identified herein. 

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.

TITLE:   GCN CIRCULAR
NUMBER:  40221
SUBJECT: LIGO/Virgo/KAGRA S250328ae: Subaru/PFS Spectroscopic  Follow up and Candidates
DATE:    25/04/24 05:12:11 GMT
FROM:    Haibin Zhang at National Astronomical Observatory of Japan (NAOJ) <haibin.zhang@nao.ac.jp>

Haibin Zhang, Mitsuru Kokubo, Nozomu Tominaga, Yousuke Utsumi, Michitoshi Yoshida (NAOJ), Tomoki Morokuma (Chiba Tech), Akira Arai, Wanqui He, Yuki Moritani, Masato Onodera, Vera Maria Passegger, Ichi Tanaka, Kiyoto Yabe (NAOJ), Sean MacBride, Isaac McMahon, Marcelle Soares-Santos (UZH), Ken Herner (Fermilab), Simran Kaur (University of Michigan/UZH), Lillian Joseph (Benedictine U.), and Tom Diehl (FNAL) report on behalf of the Japanese Collaboration for Gravitational-Wave Electro-Magnetic Follow-up (J-GEM), Subaru Telescope, and Dark Energy Survey Gravitational Wave (DESGW) Team:

Between 05:11 and 10:20 UTC on April 3, 2025, we carried out spectroscopic observations with the Subaru/Prime Focus Spectrograph (PFS) following the LVK alert issued for the gravitational-wave event S250328ae (GCN 39898). The Subaru/PFS is a fiber spectrograph capable of observing ~2400 targets simultaneously within a ~1.25 square degree field of view, and covers a wavelength range of 380 nm to 1260 nm with a resolving power of ~2500-5500 (Sugai et al. 2015; Tamura et al. 2024). We observed seven pointings centered on the ICRS coordinates listed below:

|RA [deg]|Dec [deg]|
| - | - |
|144.130530|10.436230|
|144.126501|11.595492|
|145.151367|9.856599|
|145.151367|11.015861|
|145.151367|12.175123|
|146.176234|11.595492|
|146.180717|12.754753|

These seven pointings cover the ~50% localization region of the S250328ae event.

We observed targets selected from transient candidates of GCN 39934 and 39992 (including candidates before and after vetting), X-ray sources from GCN 39972, and potential host galaxies from GLADE+ (Dálya et al. 2022) and PS1-STRM (Beck et al. 2021) catalogs in our pointings. A total of ~3900 targets were observed with an on-source exposure time of 1800 seconds in six pointings and 900 seconds in one pointing (shortened due to bad weather conditions).

We reduced the data on-site using the quick reduction system based on the PFS Data Reduction Pipeline, and then carried out classification (fitting galaxy, QSO, star, and supernova templates) and visual inspection. After classification and visual inspection, we obtained confident spectroscopic redshifts of ~70% of our targets. Among these sources, five candidates listed below were identified to be the possible electromagnetic counterpart to the gravitational-wave event S250328ae (within the ~90% three-dimensional localization volume partly covered by our pointings). Because these five candidates were originally selected from the GLADE+ and PS1-STRM catalogs that do not contain information of variability, we encourage follow-up observations to confirm their variability. 

|ID_PFS|ATNAME|RA [deg]|Dec [deg]|Type_PFS|Redshift|
| - | - | - | - | - | - |
|761|AT2019uib|145.718857|12.412288|QSO|0.130|
|9238|N/A|145.412644|12.717089|QSO|0.096|
|13647|N/A|144.989553|10.636089|QSO|0.136|
|17165|N/A|145.733559|11.593362|QSO|0.120|
|19826|N/A|145.351893|12.503455|QSO|0.136|

No counterparts to X-ray sources (GCN 39972) are recovered within their reported error bounds by our observations. We do not identify any transient candidates (GCN 39934 and 39992) we observed within the ~90% three-dimensional localization volume of S250328ae. These transient candidates falling outside the ~90% localization volume are listed below.

|ID_ DESGW|ATNAME|RA [deg]|Dec [deg]|Type_ DESGW|Type_PFS|Redshift|
| - | - | - | - | - | - | - |
|2290036|AT2025gek|145.199481|10.828527|SN_LIKE|SN Ic|0.233|
|2290467|AT2020woa|144.388798|10.255536|SN_LIKE|SN Ib|0.050|
|2292782|AT2025geo|144.928578|10.349243|SN_LIKE|SN Ia|0.182|
|2290334|AT2025geq|145.235616|12.469694|SN_LIKE|SN Ia|0.186|
|2290623|AT2025gen|144.112464|11.641817|AGN_LIKE|SN II|0.204|
|2291223|N/A|146.389563|13.131801|AGN_LIKE|QSO|0.540|
|2293190|AT2025cvb|144.191297|10.828462|SN_LIKE|SN Ia|0.140|
|2290143|AT2025ggv|146.033906|11.088812|SN_LIKE|QSO|1.437|
|2292040|AT2025ges|144.438216|9.885617|AGN_LIKE|SN Ia|0.434|
|2290059|N/A|144.679961|10.018211|AGN_LIKE|QSO|0.209|
|2292786|N/A|144.037664|10.810454|AGN_LIKE|SN Ia|0.254|
|2293517|AT2025geu|145.956024|12.381673|AGN_LIKE|QSO|0.507|
|2295862|N/A|145.361115|12.521385|AGN_LIKE|QSO|0.969|
|2290563|N/A|145.149074|11.756403|AGN_LIKE|QSO|0.255|
|2289995|AT2025ggx|144.57373|10.274445|SN_LIKE|SN II|0.358|

We are grateful to the staffs at NAOJ and Subaru Telescope for their contributions to the deployments of PFS hardware and software, and the preparations of PFS system integration, engineering observations, and various other engineering works. Our thanks should also be propagated to the administrative staffs at Kavli IPMU, NAOJ, Subaru Telescope, and all the PFS institutes for kind supports in such aspects as finances, contracts, asset managements, and so on. 

This research is based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has the cultural, historical, and natural significance in Hawaii.

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.

TITLE:   GCN CIRCULAR
NUMBER:  40455
SUBJECT: LIGO/Virgo/KAGRA S250328ae: DECam DESGW Candidates (Final Epochs)
DATE:    25/05/13 17:45:36 GMT
FROM:    Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>

Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, Tom Diehl (Fermilab), Haibin Zhang, Mitsuru Kokubo, Nozomu Tominaga, Yousuke Utsumi, Michitoshi Yoshida (NAOJ), Tomoki Morokuma (Chiba Tech), Akira Arai, Wanqui He, Yuki Moritani, Masato Onodera, Vera Maria Passegger, Ichi Tanaka, Kiyoto Yabe (NAOJ) report on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team, the Japanese Collaboration for Gravitational-Wave Electro-Magnetic Follow-up (J-GEM), and Subaru Telescope:

At 01:20 UTC April 6th and 00:14 UTC April 25th, the Dark Energy Camera (DECam) began the third and fourth epochs of observations in final response to the LVK alert issued for the candidate gravitational-wave event S250328ae (GCN 39898). The pointings of these observations were identical to the first epoch observations from March 29th, 2025 (GCN 39934). All fields were observed in DECam r, i, and z filters with 90-second exposures. The limiting magnitude achieved is ~21.3 in r-band, ~21.2 in i-band, and ~21.0 in z-band for the third epoch and ~22.3 in r-band, ~22.1 in i-band, and ~21.6 in z-band for the fourth.

We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts, requiring an autoscan score of at least 0.7 on at least 3 nights of observations. We also match our candidates against the ALLWISE, Milliquas, Quaia, and LQAC-6 AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024, Souchay et al 2024) within the LVK localization volume. Of the 88 AGNs which exhibited transient variability in our observations, none lay within the localization volume.

Of the 25 high confidence candidates reported previously (GCN 39992), 15 were observed by the J-GEM collaboration, using the Subaru Telescope Prime Focus Spectrograph (PFS), and ruled out by spectral identification and redshift (GCN 40221). Another 8 candidates either did not exhibit any further transient activity after the first two epochs or were determined to be likely stellar in origin. The final 2 candidates were not within the footprint observed by J-GEM and thus could not be determined. We report these two candidates and one last candidate which was observed only in the most recent epochs below, all likely of supernova origin.

| TYPE | ID | ATNAME | RA | DEC | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |
| -------- | ------- | --------- | ---------- | ---------- | ----- | ---- | ----- | ---- | ----- | ---- |
| SN_LIKE | 2290001 | AT2025avp | 144.806008 | 10.632223 | 20.36 | 0.02 | 20.19 | 0.02 | 20.22 | 0.06 |
| SN_LIKE | 2291473 | AT2025gem | 144.706509 | 11.560317 | 20.92 | 0.03 | 21.03 | 0.15 | 21.22 | 0.10 |
| SN_LIKE | 2295351 | AT2025kjv | 143.077064 | 7.385021 | 22.3 | 0.10 | 21.81 | 0.07 | 22.86 | 0.44 |

Additionally, J-GEM reported 5 QSOs which had a redshift consistent with the localization volume. We do not find any evidence of any transient variability for any of these QSOs in our observations after the detection of S250328ae. We also do not recover any of the X-ray source candidates reported by Swift XRT (GCN 39972) within their reported error bounds. Thus, apart from the possibility that the three supernova-like candidates reported above are related to S250328ae, we find no suitable optical counterpart candidate for this binary black hole gravitational wave event.

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations. We also thank the J-GEM and Swift XRT teams for their contribution and support.
	 		 		 	 	 		
We are grateful to the staff at NAOJ and Subaru Telescope for their contributions to the deployments of PFS hardware and software, and the preparations of PFS system integration, engineering observations, and various other engineering works. Our thanks should also be propagated to the administrative staffs at Kavli IPMU, NAOJ, Subaru Telescope, and all the PFS institutes for kind support in such aspects as finances, contracts, asset managements, and so on.
This research is based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawaii.

TITLE:   GCN CIRCULAR
NUMBER:  39871
SUBJECT: LIGO/Virgo/KAGRA S250326y: Identification of a GW compact binary merger candidate
DATE:    25/03/26 02:38:50 GMT
FROM:    Chia-Hsuan Hsiung <sw56540@gmail.com>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250326y during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-03-26 01:54:06.153 UTC (GPS time: 1426989264.153). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S250326y is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.6e-12 Hz, or about one in 1e4 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250326y

After parameter estimation by RapidPE-RIFT [7], the classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], distributed via GCN notice about 31 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [9], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1687 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 6237 +/- 1766 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [4] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [7] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002 (2015) doi:10.1103/PhysRevD.92.023002
 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  39874
SUBJECT: LIGO/Virgo/KAGRA S250326y: Updated Sky localization
DATE:    25/03/26 15:19:25 GMT
FROM:    A. Domiciano at Côte d’Azur Observatory <armando.domiciano@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250326y (GCN Circular 39871). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250326y

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 1460 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 5412 +/- 1720 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  39776
SUBJECT: LIGO/Virgo/KAGRA S250319bu: Identification of a GW compact binary merger candidate
DATE:    25/03/19 07:43:35 GMT
FROM:    sreedevi_p230198ph@nitc.ac.in

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250319bu during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-03-19 06:25:36.479 UTC (GPS time: 1426400754.479). The candidate was found by the GstLAL [1], MBTA [2], and SPIIR [3] analysis pipelines.

S250319bu is an event of interest because its false alarm rate, as estimated by the online analysis, is 4.7e-10 Hz, or about one in 67 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250319bu

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

The LIGO Hanford detector lost its working point about two seconds after the merger time. That has been carefully investigated by the LVK rapid response team and should have no impact on the significance of the signal nor on its classification or skymap.

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [4] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [4] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

Three sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [5], distributed via GCN notice about 23 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], distributed via GCN notice about 28 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [5], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 2057 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4372 +/- 1308 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [2] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
 [3] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [4] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [5] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  39780
SUBJECT: LIGO/Virgo/KAGRA S250319bu: Updated Sky localization
DATE:    25/03/19 13:55:10 GMT
FROM:    Aditya Vijaykumar <aditya.vijaykumar@ligo.org>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250319bu (GCN Circular 39776). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250319bu

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 989 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4713 +/- 1546 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040

TITLE:   GCN CIRCULAR
NUMBER:  39582
SUBJECT: LIGO/Virgo/KAGRA S250304cb: Identification of a GW compact binary merger candidate
DATE:    25/03/04 06:58:51 GMT
FROM:    Pan Guo at KAGRA <panguocas@gmail.com>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250304cb during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-03-04 06:22:45.568 UTC (GPS time: 1425104583.568). The candidate was found by the GstLAL [1] and MBTA [2] analysis pipelines.

S250304cb is an event of interest because its false alarm rate, as estimated by the online analysis, is 5.6e-08 Hz, or about one in 6 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250304cb

The classification of the GW signal, in order of descending probability, is BBH (96%), Terrestrial (4%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [3] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 2%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 28 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1569 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1768 +/- 541 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
 [2] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  39691
SUBJECT: LIGO/Virgo/KAGRA S250304cb: Upper limits from Swift/BAT-GUANO
DATE:    25/03/12 20:51:58 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 97.58% of the GW localization probability ([bayestar.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250304cb/files/bayestar.multiorder.fits)) at merger time. A fraction 43.22% of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.
In units of 10^-7 erg/s/cm^2:

|time_bin (s) |soft  |normal|hard  |GRB170817
|-|-|-|-|-|
|0.256        |7.85  |5.54  |5.01  |6.09  
|1.024        |3.99  |2.82  |2.55  |3.09  
|4.096        |2.12  |1.50  |1.36  |1.65  
|16.384       |1.29  |0.91  |0.82  |1.00  


The upper limits as function of sky position are plotted here, alongside the GW localization:
[https://zenodo.org/records/15008852]

The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: [https://www.swift.psu.edu/guano/]

TITLE:   GCN CIRCULAR
NUMBER:  39506
SUBJECT: LIGO/Virgo/KAGRA S250227e: Identification of a GW compact binary merger candidate
DATE:    25/02/27 00:59:12 GMT
FROM:    chl20171@outlook.com

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250227e during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-02-27 00:12:45.474 UTC (GPS time: 1424650383.474). The candidate was found by the GstLAL [1] analysis pipeline.

S250227e is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.6e-12 Hz, or about one in 1e4 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250227e

After parameter estimation by RapidPE-RIFT [2], the classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

There is evidence for excess noise in L1 coinciding with the merger time. There is evidence for nonstationary noise in V1 around the event time. These data quality issues may affect the parameters or the significance of the candidate.

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [3] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 26 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 500 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2077 +/- 642 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
 [2] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002 (2015) doi:10.1103/PhysRevD.92.023002
 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  39523
SUBJECT:  LIGO/Virgo/KAGRA S250227e: Updated Sky localization
DATE:    25/02/27 17:40:48 GMT
FROM:    ethan.payne@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250227e (GCN Circular 39506). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250227e

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 280 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(19h19m, +06d18m, 14.62d, 6.11d, 101.52d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2004 +/- 527 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. (2023) arXiv:2307.13380

TITLE:   GCN CIRCULAR
NUMBER:  39504
SUBJECT: LIGO/Virgo/KAGRA S250226dl: Identification of a GW compact binary merger candidate
DATE:    25/02/26 23:30:23 GMT
FROM:    chl20171@outlook.com

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S250226dl during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2025-02-26 22:48:38.699 UTC (GPS time: 1424645336.699). The candidate was found by the CWB [1], CWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S250226dl is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.2e-12 Hz, or about one in 1e4 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250226dl

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

Two sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN notice about 28 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1479 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3587 +/- 1101 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
 [4] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

TITLE:   GCN CIRCULAR
NUMBER:  39522
SUBJECT: LIGO/Virgo/KAGRA S250226dl: Updated Sky localization
DATE:    25/02/27 17:34:46 GMT
FROM:    ethan.payne@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S250226dl (GCN Circular 39504). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250226dl

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 1218 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3361 +/- 1041 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. (2023) arXiv:2307.13380