TITLE: GCN CIRCULAR NUMBER: 43586 SUBJECT: GRB 260131A/B: GOTO candidate optical counterpart consistent with the MAXI localisation DATE: 26/01/31 23:32:35 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, D. O’Neill, 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, R. Starling, B. Godson, T. Killestein, A. Kumar, M. Pursiainen, 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 GRB 260131A (Fermi/GBM Team, GCN 43579), now associated with GRB 260131B (Sugai et al., GCN 43580; Roberts et al., GCN 43585). Observations covering the initial GBM localisation area of GRB 260131A began at 2026-01-31 20:00:48 UT (+13.35h post trigger) and continued through to 2026-01-31 21:32:06 UT (+14.88h post trigger). 103 images were taken, across 10 unique pointings, covering 215.8 within the 90% localisation contour. ~89.8% of the total 2D localisation probability was covered, with an average 5-sigma depth of 19.4 mag in the GOTO-L filter (400 - 700nm). 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. A new optical source GOTO26akj/AT2026bwg is identified at coordinates: RA,DEC (J2000) = 24.713819, 34.328119, 01:38:51.32, +34:19:41.23 This position is consistent with the MAXI localisation of GRB 260131B (Sugai et al., GCN 43580). The source was initially detected with L = 17.41 ± 0.03 AB mag (t0+13.62h) before fading to L = 17.59 ± 0.04 mag (t0+14.75h). We find no evidence of the source prior to the GRB trigger time in the most recent pre-GRB GOTO observations taken at 19:59:53 on 2026-01-28 (t0-58.66h) down to a 5-sigma depth of L > 19.55 AB mag. We also find no evidence of the source prior to the GRB in the ZTF observations provided by the Lasair broker (Smith et al. 2019) or the ATLAS forced photometry server (Shingles et al. 2021). Due to its temporal coincidence with GRB 260131A/B, apparent fading, and spatial coincidence with the relatively small MAXI localisation region, we consider GOTO26akj/AT2026bwg to be a promising candidate optical counterpart. Further observations to determine the nature of the source are strongly encouraged. 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/43586. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...