TITLE: GCN CIRCULAR NUMBER: 39062 SUBJECT: GRB 250128B: GOTO optical upper limit DATE: 25/01/28 19:52:11 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> K. Ulaczyk, D. O'Neill, G. Ramsay, B. P. Gompertz, R. Starling, M. Kennedy, K. Ackley, M. J. Dyer, J. Lyman, A. Kumar, F. Jimenez-Ibarra, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle and D. Pollacco report on behalf of the GOTO collaboration: The Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) covered the field of Fermi and Swift triggered GRB 250128B (GCN 39057, 39058) at 2025-01-28 17:36:35 UT (+1.23h post-trigger). The observation was taken by GOTO-South, and 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. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues. We do not identify any candidate optical counterparts within the Swift XRT localisation uncertainty region, down to a 3-sigma limiting magnitude of L > 20.4 AB mag, consistent with the non-detections reported by Izzo et al. (GCN 39059) and Lipunov et al. (GCN 39061). Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. GOTO (https://goto-observatory.org<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/39062. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...