TITLE:   GCN CIRCULAR
NUMBER:  46073
SUBJECT: GRB 261029A: GOTO optical counterpart candidate
DATE:    26/10/29 02:20:34 GMT
FROM:    d.s.oneill@bham.ac.uk

D. O'Neill, R. Starling, B. Gompertz, 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, 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  Fermi/GBM alert GRB 261029A (Fermi GBM team, GCN #46071, SVOM/ECLAIRS, Goto et al. GCN #46072).
Observations covering the localisation area began at 2026-10-29 01:32:25 UT, (+0.09h post trigger) and continued through to 2026-10-29 01:46:56 UT (+0.33h post trigger). 23 images were taken, across 3 unique pointings, covering 91.3 sq. deg within the 90% localisation contour. ~40.5% of the total 2D localisation probability was covered, with an average 5-sigma depth of 19.8 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 GOTO26llt/AT2026ahaw is identified within the GBM 90% localisation region with co-ordinates:

    RA,DEC (J2000) = 8.817988, 34.500138,
                    00:35:16.32, +34:30:00.50

This position is 295” from the SVOM position. The source was detected with L = 13.69 ± 0.01 AB mag at 01:39:12 (+0.20h) and faded to L = 14.85±0.01 AB mag at 01:50:10 UTC (+0.386h) . We find no evidence of the source prior to the GRB trigger time in previous GOTO observations 2026-10-28 23:49:04 (-1.63h) down to a depth of >19.34 AB mag, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021).