B. P. Gompertz; M. R. Kennedy; D. O'Neill; K. Ulaczyk; T. Killestein; K. Ackley; D. B. Malesani; R. Starling; M. J. Dyer; J. Lyman; F. Jimenez-Ibarra; A. Kumar; D. Steeghs; D. K. Galloway; 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 230918B (Fermi GBM team, GCN 34734). Three targeted observations were performed by GOTO-South between 09:15:23 UT and 11:31:06 UT on 2023-09-19, corresponding to 0.39, 0.44 and 0.48 days after trigger. Each observation consisted of 4x90 s 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 a fast-rising optical source within the GBM 90% localisation region:
Name | RA(J2000) | Dec(J2000)
GOTO23aky | 20:54:37.14 | -48:40:29.33
The L-band AB magnitudes at each epoch are measured to be 20.34 +/- 0.16, 20.18 +/- 0.11, and 20.03 +/- 0.10, respectively. This corresponds to a rise of 3.3 +/- 2.0 mag/day.
We find no evidence of this source prior to the GRB trigger time in previous GOTO observations (taken ~6 days prior to the GRB) or the ATLAS forced photometry server (Shingles et al. 2021, taken ~ 1 day prior to the GRB). No underlying source is present at the position in the Legacy Server (Dey et al. 2019).
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are 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).