TITLE: GCN CIRCULAR NUMBER: 41847 SUBJECT: GRB 250916A: GOTO optical counterpart candidate DATE: 25/09/17 06:44:08 GMT FROM: Sergey Belkin at Monash University <sergey.belkin@monash.edu> S. Belkin, D. O'Neill, B. P. Gompertz, R. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, 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 GRB 250916A (Fermi GBM team, GCN 41839; Arya et al., GCN 41843). Targeted observations were performed beginning at 2025-09-16 23:39:53 UT (+10.18h post trigger) and continued through to 2025-09-17 04:55:45 UT (+15.44h post trigger). 211 images were taken, across 10 unique pointings, covering 422.1 sq deg within the 90% localisation contour. ~68.1% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.0 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. A new optical source, GOTO25hed, is identified within the GBM 90% localisation region with coordinates (J2000) RA = 26.570161, DEC = 36.16543. The source was initially detected with magnitude L = 18.90 ± 0.11 AB mag (+10.43h post trigger), before fading to L = 19.28 ± 0.12 AB mag (+14.47h post trigger). In total, the source was detected 3 times in our observations. The observed decay rate across the 4 hours our observations span is ~t^(-1.05±0.03). ATLAS forced photometry (Shingles et al. 2021) reveals a non-detection down to a 5-sigma upper limit of o > 19.6 AB mag at 2025-09-16 11:50:38 UT (-1.65h before trigger). We find no evidence of the source prior to the GRB trigger time in previous GOTO or ATLAS observations. A summary of our observations is presented below. +-----------------------------------------------------------------------------+ | Observation time (UT) | t-t0 | filter | AB mag | +-----------------------------------------------------------------------------+ | 2025-09-15 05:57:16 | -31.53h | L | >19.00 | | 2025-09-16 23:55:24 | +10.43h | L | 18.90 ± 0.11 | | 2025-09-17 02:49:13 | +13.33h | L | 19.17 ± 0.07 | | 2025-09-17 03:57:43 | +14.47h | L | 19.28 ± 0.12 | +-----------------------------------------------------------------------------+ 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 and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/41847. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...