TITLE: GCN CIRCULAR NUMBER: 44864 SUBJECT: GRB 260607A: GOTO tentative optical counterpart candidate GOTO26fqf / AT2026ong DATE: 26/06/08 09:15:55 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, M. Shrestha, R. Starling, A. Kumar, G. Ramsay, 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, D. O'Neill and M. Pursiainen report 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 short GRB 260607A (Fermi GBM Team, GCN 44855). Targeted observations from GOTO-N covering the localisation area began at 2026-06-07 23:38:04 UT, (+10.81h post trigger) and continued through to 2026-06-08 03:49:33 UT (+15.0h post trigger). 260 images were taken, across 11 unique pointings, covering 417.3 sq. deg within the 90% localisation contour. ~82.7% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.5 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. We identify one candidate optical counterpart contained within the 90% probability contour: +--------------------------------------------------------------------------+ | Internal name | IAU name | RA (J2000) | dec (J2000) | AB mag | +--------------------------------------------------------------------------+ | GOTO26fqf | AT2026ong | 314.468601 | 34.277287 | 19.54 ± 0.10 | +--------------------------------------------------------------------------+ We find no evidence of this source prior to the GRB trigger time in the previous GOTO epoch, taken at 04:13:23 UT on 2026-06-07 (t0 - 8.6h) to a 3-sigma limiting magnitude of L > 20.1 (AB). We also find no evidence for the source prior to the GRB in the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). Its position is on the 63% probability contour of the GBM localisation map. GOTO26fqf / AT2026ong was detected in 3 epochs with magnitudes of L = 19.54 ± 0.10 (t0 + 12.1h), L = 19.83 ± 0.14 (t0 + 13.4h) and L = 19.84 ± 0.14 (t0 + 14.7h), which is consistent with a power-law temporal decay with an index of t^[-1.60 ± 0.64]. However, we caution that epochs 2 and 3 are consistent with no evolution. The candidate counterpart is spatially coincident with an extended object in PanSTARRS archival imaging, which we suggest is the host galaxy of the unidentified transient. We are unable to robustly relate GOTO26fqf / AT2026ong to GRB 260607A based on available observations but further follow-up, especially spectroscopic classification, is encouraged. 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/44864. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...