TITLE: GCN CIRCULAR NUMBER: 40034 SUBJECT: GRB 250403A: GOTO optical counterpart follow-up observations DATE: 25/04/03 19:34:36 GMT FROM: Yashaswi Julakanti at University of Leicester <skyj1@leicester.ac.uk> Y. Julakanti, A. Kumar, B. P. Gompertz, D. O'Neill, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration: We report on further observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) on GRB 250403A (Julakanti et al. 2025, GCN 40026; Fermi GBM team, GCN 40025). Images were processed immediately after acquisition using the GOTO pipeline. Observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm). We continue to detect the optical counterpart (Starling et al., GCN 40028; Wu et al., GCN 40029; Du et al., GCN 40031; Li et al., GCN 40032) with the following AB magnitudes: +-------------------------------------------------------------------+ | Observation time (UT) | t-t0 | filter | mag (AB) | +-------------------------------------------------------------------+ | 2025-04-03 16:20:24 | +1.08h | L | 18.99 ± 0.07 | | 2025-04-03 16:27:28 | +1.2h | L | 19.10 ± 0.07 | | 2025-04-03 16:34:48 | +1.32h | L | 19.30 ± 0.08 | | 2025-04-03 17:41:22 | +2.44h | L | 19.35 ± 0.08 | | 2025-04-03 17:51:00 | +2.60h | L | 19.47 ± 0.12 | +-------------------------------------------------------------------+ Our observations can be fit with a broken power-law model with slopes of t^-0.90 and t^-0.44 on either side of a break at 1.32 hours. 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 and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/40034. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...