TITLE: GCN CIRCULAR NUMBER: 40654 SUBJECT: GRB 250529A: GOTO optical counterpart candidate (GOTO25dea/AT 2025ndt) DATE: 25/06/09 11:53:52 GMT FROM: Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK amitkundu515@gmail.com

A. Kumar, D. O’Neill, B. P. Gompertz, G. Ramsay, R. Starling, S. Belkin, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the 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 detected GRB 250529A (GBM770183181; Fermi GBM Team, GCN 40567).

Targeted observations were performed with GOTO-North beginning at 2025-05-29 03:59:57 UT (+0.23h post trigger) and continued through to 2025-05-29 04:49:24 UT (+1.05h post trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm).

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.

Following the refinement of the GRB localisation (Kozyrev et al., GCN 40629), we reviewed our sources consistent with the new IPN annulus. We identify a new transient at RA, Dec = 18.071949, 58.347004 (equivalent to RA = 01:12:17.27 and Dec = +58:20:49.21), contained within the annulus. The source was initially detected with a magnitude of L = 18.76 ± 0.28 mag (+0.90h after the GBM trigger). The second epoch, taken at +1.05h post trigger, shows marginal evidence for fading, with a measured magnitude of L = 19.14 ± 0.12 mag. Observations taken on the following night (t0+25h) yield only a 3-sigma upper limit of L > 19.05 mag.

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).

While the candidate lies close to the Galactic plane (latitude -4.4 degrees) and has only weak evidence for fading, the significant reduction of the localisation area now makes it a much stronger (but still uncertain) afterglow candidate for GRB 250529A.

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 and the Instituto de Astrofisica de Canarias (IAC).

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