TITLE: GCN CIRCULAR NUMBER: 39455 SUBJECT: EP250223a: GOTO optical afterglow detections DATE: 25/02/24 13:59:07 GMT FROM: d.s.oneill@bham.ac.uk D. O’Neill, B. P. Gompertz, A. J. Levan, K. Ulaczyk, G. Ramsay, A. Kumar, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, F. Jimenez-Ibarra, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Pall'e 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 EP/WXT alert WXT01709131863 (Lian et al. GCN 39429). Three epochs of targeted observations were performed: at 2025-02-24 09:56:36 UT (+18.9h post trigger), 2025-02-24 11:04:37 UT (+20.0h post trigger), and 2025-02-24 12:12:37 UT (+21.1h post trigger). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings. We detect the optical afterglow (Hauptmann et al., GCN 39436; Levan et al., GCN 39438; Wu et al., GCN 39439; Pérez-Fournon et al., GCN 39440; Izzo et al., GCN 39441; Xin et al., GCN 39445; Guo et al., GCN 39447; An et al., GCN 39449; Brivio et al., GCN 39450; Ducoin et al., GCN 39453) with magnitudes of L (400-700 nm) = 20.04 ± 0.15 mag (+18.9h), L = 19.78 ± 0.11 mag (+20.0h) and L = 19.97 ± 0.14 mag (+21.1h). Our measurements indicate no fading of the afterglow between 18 and 21 hours after trigger, continuing the shallow evolution noted by Ducoin et al. during the first 6 - 15 hours (GCN 39453). 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/39455. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...