LIGO/Virgo/KAGRA S250725j

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
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  (Hall et al. GCN 41177
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 , McMahon et al. GCN 41206
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 ) 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
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  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
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 , GCN 41206
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 .

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).

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
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 ). 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
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  and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
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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
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  and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
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 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
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 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
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