LIGO/Virgo/KAGRA S250727dc

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
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  (GCNs 41179
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 ) of the gravitational-wave event  S250727dc (GCNs 41179
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 , 41487
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 ), reported by Swift/UVOT (GCN 41179
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 ,GCN 41187, GCN 41188
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 , GCN 41196
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 , GCN 41197
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 , GCN 41201
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 , GCN 41202
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 , GCN 41205
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 , GCN 41211
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  and GCN 41214
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 ). 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
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 ).
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).

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
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  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.

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
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 ). 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.

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
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 ). 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
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  and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
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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.

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