TITLE: GCN CIRCULAR NUMBER: 38684 SUBJECT: GRB 241228B: GOTO possible optical counterpart candidate DATE: 24/12/28 08:40:34 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com> A. Kumar, S. Belkin, B. P. Gompertz, D. O'Neill, G. Ramsay, 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 optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al. 2024) in response to the Fermi/GBM detected GRB 241228B (alert GBM757051990; Fermi GBM Team, GCN 38682). Targeted imaging obtained with GOTO-North covered the Fermi localisation region from 2024-12-24 04:26:19 UT (+0.22h post trigger) to 2024-12-24 06:05:06 UT (+1.87h post trigger). In total, 88 images were taken, across 10 unique pointings, covering 205.5 square degrees within the 90% localisation contour. ~87.4% of the total 2D localisation probability was covered, with an average 5-sigma depth of 19.4 mag. The observations 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. 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, GOTO24jmz, is identified. It is located on the 94.5% probability contour, formally outside of the GBM 90% localisation region. The source is initially detected at a magnitude of L = 14.54 ± 0.01 at 04:32:24 (t0+19 minutes post-trigger) before decaying to L = 16.99 ± 0.04 mag at 05:43:42 (t0+90 minutes post-trigger). This fading of 2.3 mags in 71 minutes is equivalent to a decay rate of t^-1.45 from the time of the GBM trigger, consistent with GRB afterglows. +------------------------------------------------------------------------------------+ | Internal name | RA (J2000) | Dec (J2000) | Time (UT) | t-t0 (minutes) | Mag | +------------------------------------------------------------------------------------+ | GOTO24jmz | 08:31:05.46 | +06:50:54.07 | 2024-12-28 04:32:24 | 19 | 14.54 ± 0.01 | | 2024-12-28 05:43:42 | 90 | 16.99 ± 0.04 | We find no evidence of the source to a 3-sigma limiting magnitude of L > 20.3 mag prior to the GRB trigger time in previous GOTO observations taken at 17:47:09 UT on 2024-12-27 (t-t0 = -10.43h pre-trigger). In addition, we find no evidence of this source prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). There is a faint, ambiguous underlying source at the position of GOTO24jmz visible in DESI legacy survey images that we cannot definitively identify as a star or galaxy. 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/38684. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...