TITLE: GCN CIRCULAR NUMBER: 44505 SUBJECT: GRB 260509A: GOTO optical counterpart candidate DATE: 26/05/09 23:23:47 GMT FROM: Sergey Belkin at Monash University <sergey.belkin@monash.edu> D. O'Neill, S. Belkin, K. Ulaczyk, A. Kumar, K. Ackley, M. Dyer, J. Lyman, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, R. Starling, B. Gompertz, B. Godson, T. Killestein, M. Pursiainen, on behalf of 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 SVOM/ECLAIRS alert GRB 260509A (sb26050905, Götz et al. GCN 44504). Observations covering the localisation area were taken at 2026-05-09 21:56:19 UT, (t0 +0.09h) with an average 5-sigma depth of 20.8 mag. Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations (Lyman et al. 2026). 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 source GOTO26emi is identified near the edge of the ECLAIRS localisation region, at an angular separation of 4.79 arcmin from the reported GRB position, marginally outside the quoted 4.72 arcmin error radius. The source coordinates: RA,DEC (J2000) = 176.899834, 0.288276, 11:47:35.96, +00:17:17.80 The rapidly rising source was detected with average L = 20.13 ± 0.16 AB mag (+0.09h). We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2026-05-09 10:38:10 (-11.21h) down to a depth of >20.40 AB mag, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). 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/44505. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...