TITLE: GCN CIRCULAR NUMBER: 43169 SUBJECT: EP 251220a: GOTO optical counterpart detection DATE: 25/12/21 11:08:01 GMT FROM: Amit Kumar at The Open University, UK <amitkundu515@gmail.com> A. Kumar, G. Ramsay, M. Kennedy, 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 EP 251220a (Liu et al. GCN 43162). The observations were conducted with GOTO-North at 2025-12-20 19:51:42 UT (11.4 hours post-trigger), consisting of 4x45s exposures in the GOTO L-band (400-700 nm). Difference imaging was performed using deeper template observations of the same pointings. We detect the optical counterpart of EP 251220a (Álvarez et al., GCN 43167) in the GOTO L-band at 3σ, with a magnitude of 20.7 ± 0.3 (AB). 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, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/43169. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...