TITLE: GCN CIRCULAR NUMBER: 44737 SUBJECT: GRB 260529A: GOTO tentative optical afterglow candidate DATE: 26/05/29 12:21:36 GMT FROM: Nusrin Habeeb at University of Leicester <nh312@leicester.ac.uk> N. Habeeb, A. Kumar, D. O’Neill, R. Starling, B. Gompertz, G. Ramsay, S. Belkin, 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, M. Pursiainen, 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 Fermi/GBM detected GRB 260529A (fermi_801715802, Fermi GBM team, GCN 44733). Observations covering the GBM localization began at 2026-05-29 03:00:27 UT (+0.18h post trigger) and continued through to 2026-05-29 04:55:50 UT(+2.10 h post trigger). Images consisted of 4x90s exposures in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline (Lyman et al. 2026) and difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. A new optical transient, GOTO26fez (AT 2026nuz), is identified within the 90% localization region of the Fermi/GBM trigger. The source was detected in the GOTO L-band with magnitudes of 17.85 ± 0.06 and 17.90 ± 0.04 AB mag at +0.23 h and +1.3 h post-trigger, respectively. We would like to caution that, given the source location close to the Galactic plane, we cannot exclude the possibility that the transient is of Galactic origin, such as a stellar flare or a CV. We find no evidence of this source prior to the GRB trigger time in previous GOTO observations, in the archival ATLAS forced photometry server (Shingles et al. 2021) to a magnitude limit of m_o > 18.06 AB mag ~26.3h prior to the trigger, nor in archival ZTF observations provided by the Lasair broker (Smith et al. 2019). We note, however, that the most recent pre-trigger observations from GOTO were obtained approximately one month prior to the burst, and therefore do not provide constraints on shorter-timescale pre-burst variability. 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/44737. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...