TITLE: GCN CIRCULAR NUMBER: 42658 SUBJECT: LIGO/Virgo/KAGRA S251112cm: Initial GOTO-South observations DATE: 25/11/12 22:47:23 GMT FROM: kendall.ackley@warwick.ac.uk K. Ackley, B. Godson, D. O'Neill, B. P. Gompertz, M. Dyer, J. Lyman, K. Ulaczyk, S. Belkin, T. Killestein, A. Kumar, M. Pursiainen, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, J. Casares Velásquez, L. K. Nuttall, report on behalf of the GOTO collaboration: We carried out optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al 2024) in response to the LIGO/Virgo/KAGRA gravitational wave candidate event S251112cm (GCN 42650). Targeted observations were performed by GOTO-South (Siding Spring, Australia) starting at 2025-11-12 16:09:51 UT, (+0.85h post trigger) and continued through to 2025-11-12 17:51:21 UT (+2.54h post trigger). A total of 39 image sets with exposures of 4x60s and 4x90s were taken across 6 unique pointings, covering 250.5 sq. degrees within the 90% localisation contour of the initial skymap. We covered approximately 14.6% of the total 2D localisation probability with an average 5-sigma depth of L~19.9 mag (AB). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. 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. Within the initial skymap, we identify 2 optical sources within the 90% contour and 1 optical source within the 99% probability contour. We find no convincing evidence of excess flux in either GOTO25jyg/AT2025addc or GOTO25jya/AT2025adcv prior to the GW trigger time in previous GOTO observations or the ZTF observations provided by the Lasair broker (Smith et al. 2019). The ATLAS forced photometry server (Shingles et al. 2021) returns recent detections for GOTO25jyg and GOTO25jya, albeit at low significance. We encourage further analysis to confirm these pre-detections. We note that GOTO25jya appears to be associated with a GLADE+ galaxy HyperLEDA 422311 with zphot~0.01 (D_L~474 Mpc) which, if correct, would put it outside the nominal luminosity distance estimates of S251112cm. We do not find a host galaxy underlying GOTO25jyg. There is a galaxy located 9.27 arcsec from the candidate’s position within the Legacy DR10 photometric redshift catalog with zphot ~ 0.3 (Dey et al. 2019). GOTO25jye/AT2025adcy is located outside of the 90% probability contour of the initial skymap and is potentially associated with a GLADE+ galaxy HyperLEDA 389528 at zphot ~ 0.13 (D_L~664 Mpc). This association would make this candidate unlikely to be related to S251112cm. | Internal name | IAU name | RA(J2000) | Dec(J2000) | Mag(AB) | | ------------- | ---------- | ----------- | ------------ |--------------- | | GOTO25jyg | AT2025addc | 04:55:26.13 | -53:09:43.61 | 19.93 +/- 0.20 | | GOTO25jya | AT2025adcv | 03:05:58.60 | -54:09:06.60 | 19.30 +/- 0.10 | | GOTO25jye | AT2025adcy | 04:35:28.56 | -57:28:31.64 | 19.71 +/- 0.12 | Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction. Continued observations are planned from both GOTO sites, weather permitting. 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, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/42658. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...