LIGO/Virgo/KAGRA S250727dc
GCN Circular 41216
Subject
LIGO/Virgo/KAGRA S250727dc: GRANDMA/OPD observations of AT2025smm
Date
2025-08-01T22:47:59Z (24 days ago)
From
Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>
Via
Web form
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).
GCN Circular 41214
Subject
LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates
Date
2025-07-31T20:21:13Z (25 days ago)
From
Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>
Via
Web form
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.
GCN Circular 41211
Subject
LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates
Date
2025-07-31T13:19:11Z (25 days ago)
From
Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>
Via
Web form
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.
GCN Circular 41205
Subject
LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm
Date
2025-07-30T20:28:25Z (a month ago)
Edited On
2025-07-31T14:19:04Z (25 days ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
email
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.
GCN Circular 41202
Subject
LIGO/Virgo/KAGRA S250727dc: Upper limits from Swift/BAT-GUANO
Date
2025-07-30T17:49:32Z (a month ago)
From
Maia Williams at PSU <mjw6837@psu.edu>
Via
Web form
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/
GCN Circular 41201
Subject
LIGO/Virgo/KAGRA S250727dc: Swift-XRT observations, 8 X-ray sources
Date
2025-07-30T10:45:29Z (a month ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
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.
GCN Circular 41197
Subject
LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS upper limits of AT 2025smm
Date
2025-07-29T20:55:32Z (a month ago)
From
xjh@andrew.cmu.edu
Via
Web form
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.
GCN Circular 41196
Subject
LIGO/Virgo/KAGRA S250727dc: PRIME J-band upper limits of AT 2025smm
Date
2025-07-29T18:11:07Z (a month ago)
From
Joe Durbak at UMD <gcn.joedurbak@gmail.com>
Via
Web form
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.
GCN Circular 41188
Subject
LIGO/Virgo/KAGRA S250727dc: Updated Sky localization
Date
2025-07-28T17:40:55Z (a month ago)
From
lucy.thomas@ligo.org
Via
Web form
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
GCN Circular 41187
Subject
LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm
Date
2025-07-28T15:31:25Z (a month ago)
Edited On
2025-07-31T14:15:47Z (25 days ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
email
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.
GCN Circular 41179
Subject
LIGO/Virgo/KAGRA S250727dc: Identification of a GW compact binary merger candidate
Date
2025-07-27T20:24:58Z (a month ago)
From
aubrey_laity@uri.edu
Via
Web form
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