TITLE: GCN CIRCULAR NUMBER: 42758 SUBJECT: EP251118a: GOTO optical afterglow detection DATE: 25/11/19 11:15:29 GMT FROM: Sergey Belkin at Monash University <sergey.belkin@monash.edu> S. Belkin, M. E. Wortley, D. O'Neill, B. P. Gompertz, G. Ramsay, R. L. C. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, M. Pursiainen report on behalf of 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 EP/WXT alert WXT11900480769 (Jiang et al., GCN 42749). Observations covering the position began on 2025-11-19 at 02:58:22 UT (+10.26 hr post trigger) and continued through to 2025-11-19 at 04:30:49 UT (+11.81 hr post trigger). Three epochs were obtained: the first through serendipitous survey coverage (4x45 s) and the second and third through targeted response (4x90 s), all 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. We detect the optical counterpart (Malesani et al., GCN 42751), consistent with the X-ray counterpart (Shi et al., GCN 42754), with L = 20.08 ± 0.21 (AB) on 2025-11-19 at 02:58:22 UT (+10.26 hr). Follow-up L-band images at 03:22:37 UT (+10.67 hr) and 04:30:49 (+11.81 hr) yield L = 20.24 ± 0.17 and 20.17 ± 0.16 (AB), respectively. 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/42758. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...