TITLE: GCN CIRCULAR NUMBER: 35805 SUBJECT: GRB 240225B: GOTO detection of a candidate optical afterglow DATE: 24/02/26 20:45:40 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, R. L. C. Starling, M. Kennedy, G. Ramsay, D. B. Malesani, B. Godson, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, A. Kumar, D. O'Neill, 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 observations of the MAXI-discovered GRB 240225B (Nakajima et al, GCN 35796) with the Gravitational-wave Optical Transient Observer (GOTO, Steeghs et al. 2022). GOTO-North serendipitously tiled the MAXI localisation at 21:45:51 UT on 2024-02-25, 1.5 hours after trigger. The observation consisted of 4x45 s exposures in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues. Human vetting was carried out in real time on any candidates that passed the above checks. We identify a new optical source (GOTO24tz/AT2024dgu) with a magnitude of L = 17.12 +/- 0.04 at RA 08:33:26.67, Dec +27:04:32.71 (J2000). The optical source is close to the probable X-ray afterglow identified in tiled Swift observations (Evans, GCN 35797), offset by 7.2”, whilst formally outside the 4.4” 90% confidence contour of the X-ray localisation. This source was not present in the previous GOTO epoch, taken one day prior at 22:05:36 UT on 2024-02-24, to a limiting magnitude of L > 18.7. Given the tight pre-burst limit and close proximity with a new X-ray source, we propose this as a strong candidate afterglow of GRB 240225B. 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/35805. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...