TITLE: GCN CIRCULAR NUMBER: 39072 SUBJECT: GRB 250129A: GOTO optical observations show brightening afterglow DATE: 25/01/29 07:30:35 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> S. Belkin, B. P. Gompertz, D. O'Neill, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, A. Kumar, K. Ulaczyk, F. Jimenez-Ibarra, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle 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 Swift detected GRB 250129A (Beardmore et al., GCN 39066). Targeted imaging obtained with GOTO-North covered the Swift localisation region starting at 2025-01-29 04:47:52 (+2.7m post trigger) with an average 5-sigma depth of 20.1 mag. The observations consisted of 4x90s 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. We detect the optical afterglow reported by UVOT, also detected by Schnieder et al. (GCN 39071), in two epochs centred at 5.2 minutes and 1.22 hours after trigger. The measured AB magnitudes are L = 17.86 ± 0.03 and L = 17.11 ± 0.02 respectively. The afterglow has therefore brightened by approximately 0.75 magnitudes over this period. 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/39072. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...