TITLE: GCN CIRCULAR NUMBER: 43641 SUBJECT: GRB 260208A: GOTO detection of a potential optical counterpart (GOTO26aob) DATE: 26/02/08 14:01:53 GMT FROM: MEW020@student.bham.ac.uk M. E. Wortley, D. O'Neill, A. Kumar, G. Ramsay, B. P. Gompertz, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, R. Starling, B. Godson, T. Killestein, 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 the Fermi/GBM GRB 260208A (trigger GBM792220053; Fermi GBM team, GCN 43638). Observations covering the localisation area began at 2026-02-08 05:19:32 UT (+0.20h post trigger) and continued through to 2026-02-08 06:31:29 UT (+1.4h post trigger). All observations were performed with 90s integrations using the GOTO L-band filter (400–700 nm). Images were processed immediately after acquisition using the GOTO pipeline. 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. We detect the source reported by R. Podesta et al. (GCN 43640) with an L-band = 13.78 ± 0.01 AB mag at 2026-02-08 05:19:32 UT (t0+0.20h) before it faded to L=16.72 ± 0.01 AB mag at 2026-02-08 06:29:16 UT (t0+1.36h). We find no evidence of this source prior to the GRB trigger time in previous GOTO observations taken at 2026-02-08 03:33:31 UT (t0-1.57h pre-trigger) down to a 5-sigma depth of L>19.07 AB mag. 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/43641. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...