TITLE: GCN CIRCULAR NUMBER: 40464 SUBJECT: GRB 250515A: GOTO candidate optical counterpart DATE: 25/05/16 09:20:14 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, R. Starling, D. O’Neill, G. Ramsay, A. Kumar, 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, 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 250515A (Fermi GBM team, GCN 40462). Targeted observations were performed by GOTO-North (La Palma) beginning at May 15 2025 21:21:20 UT, (+0.58h post trigger) and continuing through to May 16 2025 00:15:21 UT (+3.48h post trigger). 232 images were taken, across 10 unique pointings, covering 471.6 square degrees within the 90% localisation contour. ~60.1% of the total 2D localisation probability was covered, with an average 5-sigma depth of 19.8 mag in the GOTO L-band (400-700nm). 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, GOTO25cqo, is identified within the GBM 90% localisation region at RA 10:38:32.5, Dec +19:25:03.1 (J2000). This position is on the 37% probability contour of the Fermi/GBM localisation map. The source was initially detected with magnitude L = 20.17 ± 0.21 mag (+0.71h after the GBM trigger), rising to L = 19.79 ± 0.15 mag at +1.85h post trigger. In a 3rd epoch, taken at t0+2.99 hours, we obtain a 3-sigma upper limit of L > 19.21. We find no strong evidence of this 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). However, we caution that observations to sufficiently constraining depths are not available in the weeks prior to the GRB. We note a marginal flux excess in two ATLAS images taken on 2025-05-01 and 2025-05-03. Visual inspection of these images does not reveal any compelling source, and no significant flux excess was seen in the 7 epochs between these dates and GRB 250515A. From available observations, we cannot confirm whether GOTO25cqo is associated with GRB 250515A, but given its time coincidence and apparent rapid evolution, we encourage further follow-up observations. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. Observations are ongoing. 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/40464. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...