TITLE: GCN CIRCULAR NUMBER: 43209 SUBJECT: GRB 251222A / EP251222b: GOTO optical counterpart detection DATE: 25/12/23 07:19:23 GMT FROM: Amit Kumar at The Open University, UK <amitkundu515@gmail.com> A. Kumar, G. Ramsay, D. O’Neill, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, 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) that serendipitously covered the field of GRB 251222A / EP251222b, detected by Fermi/GBM (The Fermi GBM team, GCN 43186), SVOM/ECLAIRs (Yang et al., GCN 43188), EP/WXT (Guo et al., GCN 43201) and SVOM/GRM (Wang et al., GCN 43202). Targeted observations were performed by GOTO-North at 2025-12-22 23:02:47 UT (5.95 hours post-trigger). The observation consisted of 4x45s 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. The optical afterglow (Palmerio et al., GCN 43189; An et al., GCN 43194; Calapai et al., GCN 43195; Durbak et al., GCN 43197; Li et al., GCN 43203; Saccardi et al., GCN 43204; Gupta et al., GCN 43207; Saccardi et al., GCN 43208) detected with an L-band magnitude of 19.51 ± 0.11 (AB). 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, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/43209. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...