| Circular_text |
TITLE: GCN CIRCULAR
NUMBER: 34878
SUBJECT: GRB 231024A: GOTO optical counterpart candidate
DATE: 23/10/25 06:23:08 GMT
FROM: Ben Gompertz at U of Birmingham
B. P. Gompertz, K. Ackley, D. K. Galloway, T. Killestein, R. Starling, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, A. Kumar, D. O'Neill, D. Steeghs, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Pall'e and D. Pollacco report on behalf of the GOTO collaboration:
We report on observations with the Gravitational-wave Optical Transient Observer (GOTO, Steeghs et al. 2022) in response to GRB 231024A (Fermi GBM Team, GCN 34876). Targeted observations were performed by GOTO-North between 22:15:31 UT on 2023-10-24 and 00:31:24 UT on 2023-10-25 (starting 0.37 days after 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 recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues. Human vetting was carried out in real time on any candidates that passed the above checks.
We identify one candidate optical counterpart within the GBM 90% localisation region that is not present 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). The most recent observation of the field was taken by ATLAS at 05:05:19 UT on 2023-10-24, 8.25 hours before trigger.
Name | RA(J2000) | Dec(J2000) | Filter | Mag(AB)
GOTO23baj | 00:55:07.30 | -15:15:19.43 | L | 18.37 +/- 0.07
The source is consistent with a galaxy in the WISExSuperCOSMOS PhotoZ SVM catalogue (Bilicki et al., 2016) with a reported redshift of z = 0.0734 +/- 0.0354 and is consistent with a power-law decay with an index of t^-0.7 across 3 epochs of 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).
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