TITLE: GCN CIRCULAR NUMBER: 38655 SUBJECT: GRB 241223A: GOTO optical observations summary DATE: 24/12/23 19:29:32 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com> A. Kumar, D. O’Neill, B. P. Gompertz, 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 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 241223A (Fermi GBM team, GCN 38650). Targeted observations were performed beginning at 2024-12-23 14:11:18, (+2.04h post trigger) and continued through to 2024-12-23 17:28:07 UT (+5.23h post trigger). 87 images were taken, across 5 unique pointings, covering 151.2 square degrees within the 90% localisation contour, distributed over three epochs. ~88.1% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.7 mag. Each observation consisted of 8x90s exposures. We also had serendipitous coverage in the all-sky survey mode which consisted of 4x45s exposures. All images were taken in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey 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. Human vetting was carried out in real time on any candidates that passed the above checks. We identified AT 2024aera/GOTO24jht (RA = 10:43:01.030 and Dec = -37:32:47.95) as a possible optical counterpart within the GBM 90% localisation region. The first epoch of GOTO24jht was obtained at 2024-12-23 14:16:04 UT (+2.12h post-trigger), which showed its magnitude to be L = 17.13 ± 0.01 (AB). We found no evidence of this source prior to the GRB trigger time in previous GOTO observations taken -44.73h pre-trigger down to a 3-sigma limiting magnitude of L > 20.0, or in the ATLAS forced photometry server (Shingles et al. 2021). Before a second epoch could be taken to confirm a decay, GOTO-south was forced to shut due to wind. Based on the available evidence, a Swift ToO was requested (Evans, GCN 38653) to confirm whether or not the source was the afterglow. However, conditions subsequently improved and GOTO-south reopened approximately 1 hour later. The second and third epochs were obtained at 2024-12-23 16:06:14 (+3.96h post-trigger) and 17:20:34 (+5.20h post-trigger) UTs. These showed the source had brightened to L = 16.84 ± 0.01 and L = 16.71 ± 0.01 AB magnitudes, respectively. Coupled with the position of the source close to the galactic plane (+18.7 degrees latitude), this suggests that AT 2024aera/GOTO24jht is likely an unrelated CV and not the afterglow of GRB 241223A. We also note 2 additional newly-reported transients (AT 2024aerb/GOTO24jhu and AT 2024aerc/GOTO24jhv) within the GBM 90% localisation region. However, these sources have exhibited fluctuating flux in both ATLAS and GOTO observations over the past week; hence these objects are unlikely to be related to GRB 241223A. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and were 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/38655. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...