TITLE: GCN CIRCULAR NUMBER: 43538 SUBJECT: GRB 260127B: GOTO candidate optical counterpart DATE: 26/01/28 10:22:31 GMT FROM: d.s.oneill@bham.ac.uk D. O’Neill, B. P. Gompertz, G. Ramsay, M. Kennedy, D. Steeghs, J. Lyman, R. Starling, K. Ackley, M. Dyer, K. Ulaczyk, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, 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 the short GRB 260127B (Fermi GBM Team, GCN 43533). Observations covering the localisation area began at 2026-01-28 00:20:53 UT (+4.31h post trigger) and continued through to 2026-01-28 05:10:59 (+9.14h post trigger). 141 images were taken, across 9 unique pointings, covering 242.7 square degrees within the 90% localisation contour. ~61.4% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.2 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, GOTO26aih (AT2026bnn), within the 90% probability contour at coordinates RA = 12:49:52.73, Dec = +66:44:58.87, equivalent to RA = 192.469711, Dec =66.74969 degrees. This position lies on the 84th percentile probability contour of the Fermi/GBM localisation map. The source was initially detected with L = 19.15 ± 0.14 AB mag (t0+5.66h), before fading to L = 19.82 ± 0.11 mag (t0+7.91h). The measured rate of decay is t^-1.66 ± 0.97. We find no evidence of the source prior to the GRB trigger time in the most recent GOTO observations taken at 2026-01-20 06:01:17 (t0-7.6 days) down to a 5-sigma depth of >20.8 AB mag. We also find no evidence for pre-GRB emission in the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). GOTO26aih/AT2026bnn is spatially coincident with the galaxy SDSS J124952.79+664500.4, offset by 1.5” from the core. This galaxy has a measured spectroscopic redshift of z = 0.12 (DESI Collaboration, 2025). A summary of our observations is presented below. +------------------------------------------------------------+ | Observation time (UT) | t-t0 | filter | mag | +------------------------------------------------------------+ | 2026-01-20 06:01:17 | -181.42h | L | >20.8 | | 2026-01-28 01:01:13 | +5.66h | L | 19.15 ± 0.14 | | 2026-01-28 02:01:23 | +6.78h | L | 19.79 ± 0.14 | | 2026-01-28 03:01:22 | +7.91h | L | 19.82 ± 0.11 | +-------------------------------------------------------------+ Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and 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/43538. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...