TITLE: GCN CIRCULAR NUMBER: 43514 SUBJECT: GRB 260125A: GOTO candidate optical counterpart DATE: 26/01/25 20:20:30 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, S. Moran, D. O’Neill, R. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, M. Pursiainen report on behalf of GOTO collaboration: We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the GRB 260125A (Fermi GBM Team, GCN 43510). Observations from GOTO-South (Siding Spring) covering the localisation area began at 2026-01-25 12:06:29 UT (+4.49h post trigger) and continued through to 2026-01-25 15:46:04 UT (+8.54h post trigger). 292 images were taken, across 16 unique pointings, covering 510.2 within the 90% localisation contour. ~78.0% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.8 mag. 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 90% probability region, we identify a new transient, GOTO26acv (AT 2026bjf), lying on the 20th percentile contour of the GBM localisation map at coordinates: RA = 10:06:16.71, Dec = -06:34:33.55 (J2000), equivalent to RA = 151.56962, Dec = -6.57599 degrees. GOTO26acv / AT 2026bjf was discovered with an AB magnitude of L = 20.17 ± 0.16 in an epoch taken at 12:29:25 UT, i.e. 4.87h post-trigger. Across three epochs of detection, it was observed to fade by 0.42 ± 0.25 magnitudes in 2.35 hours, equivalent to a power-law decay rate of t^-0.92 ± 0.38. The final detection magnitude was L = 20.59 ± 0.19 at 14:50:26 UT. The source is spatially coincident with the galaxy PGC 139241 at a luminosity distance of 53 Mpc. It is offset from the nucleus by ~9”. The source was not present in the most recent survey image of the field, taken at 13:37:58 UT on 2026-01-24 (0.75 days prior to the GRB trigger), to a 3-sigma limiting magnitude of L > 20.95. We also find no evidence of GOTO26acv / AT 2026bjf prior to the GRB trigger time in the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). Further follow-up observations to establish the nature of GOTO26acv / AT2026bjf and its potential association with GRB 260125A are encouraged. 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, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/43514. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...