TITLE: GCN CIRCULAR NUMBER: 44903 SUBJECT: GRB 260610B: GOTO optical counterpart candidate DATE: 26/06/11 02:49:52 GMT FROM: d.s.oneill@bham.ac.uk D. O'Neill, J. Mills, M. Pursiainen, J. Lyman, K. Ackley, M. Dyer, K. Ulaczyk, 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, A. Kumar, 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 Fermi/GBM alert GRB 260610B (GCN #44901). Observations covering the localisation area began at 2026-06-10 23:58:04 UT, (+0.2h post trigger) and continued through to 2026-06-11 00:31:39 UT (+0.76h post trigger). 63 images were taken, across 10 unique pointings, covering 241.5 sq. deg within the 90% localisation contour. ~89.6% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.6 mag. Images were processed immediately after acquisition using the GOTO pipeline. 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 (Lyman et al. 2026). A new optical source GOTO26fua/AT2026owq is identified within the GBM 90% localisation region (25% contour) with co-ordinates: RA,DEC (J2000) = 218.159414, 27.004935, 14:32:38.26, +27:00:17.76 The source was detected with L = 16.66 ± 0.01 AB mag (+0.21h) before fading to L = 17.83 ± 0.03 AB mag (+1.4h) with the light curve consistent with a decay index t^-0.56. We find no evidence of the source prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021) at m_o > 19.45 mag (-38.99h). The temporal and spatial coincidence along with the power-law like decay strongly suggests this is the optical counterpart of GRB 260610B. 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, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/44903. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...