TITLE: GCN CIRCULAR NUMBER: 42040 SUBJECT: GRB 251001B: GOTO optical counterpart detection DATE: 25/10/01 15:09:24 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, D. O'Neill, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar, G. Ramsay, 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 observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 251001B (Fermi GBM team, GCN 42038; Beardmore et al. GCN 42039). Targeted observations were performed at 2025-10-01 14:33:37 (starting 16.3 minutes after trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings. At a position consistent with the Swift/XRT localisation, we detect a new optical source, GOTO25icy, with L = 19.33 ± 0.12 AB mag, at coordinates: RA,DEC (J2000) = 42.651328, -23.02001 | 02:50:36.32, -23:01:12.03 We find no evidence of this source prior to the GRB trigger time in the previous GOTO observation taken at 2025-10-01 14:01:01 UT (16.3 min pre-trigger) down to a 3-sigma depth of L > 20.1 AB mag. We note the source is coincident (~0.4”) with a faint g~25 AB mag source in Legacy Survey DR10, which may be the host galaxy of the GRB. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. Observations are ongoing. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/42040. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...