GCN Circular 46073

Subject

GRB 261029A: GOTO optical counterpart candidate

Event

GRB 261029A

Date

2026-10-29T02:20:34Z (4 days ago)

Edited On

2026-10-29T14:06:37Z (3 days ago)

From

d.s.oneill@bham.ac.uk

Edited By

Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of d.s.oneill@bham.ac.uk

Via

Web form

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 #46071Loading...
 
 , SVOM/ECLAIRS, Goto et al. GCN #46072Loading...
 
 ).
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/AT2026ahawLoading...
 
  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).