TITLE: GCN CIRCULAR NUMBER: 38691 SUBJECT: GRB 241228B: GOTO24jmz optical follow-up observations DATE: 24/12/28 17:51:08 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com> A. Kumar, G. Ramsay, B. P. Gompertz, S. Belkin, D. O'Neill, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, 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: We report on updated observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) on GRB 241228B (Fermi GBM Team, GCN 38682). Follow-up observations of the GOTO optical counterpart candidate GOTO24jmz (Kumar et al. GCN 38684) were performed by GOTO-South at 2024-12-28 13:00:42 UT (t0+8.79h post-trigger). The observation consisted of 4x90s exposure 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. GOTO24jmz (RA = 08:31:05.46 and Dec = +06:50:54.07) was detected at an L-band magnitude of 19.73 ± 0.10 at t0+8.79h post-trigger. These updated observations align well with the previously reported decay rate of t^−1.45 (Kumar et al., GCN 38684) and are also consistent with the magnitude reported by An et al. (GCN 38687). We encourage further deeper follow-up observations. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are 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 and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/38691. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...