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GCN Circular 34681

GRB 230911A: GOTO optical counterpart candidate
2023-09-13T10:07:56Z (9 months ago)
Sergey Belkin at Monash University <>
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S. Belkin; B. P. Gompertz; A. Kumar; K. Ackley; K. Wiersema; D. O’Neill; T. Killestein; R. Starling; M. J. Dyer; J. Lyman; K. Ulaczyk; F. Jimenez-Ibarra; 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 230911A (Fermi GBM team, GCN 34652). Observations were performed by GOTO North and South between 04:19:47 UT on 2023-09-11 and 17:08:39 UT on 2023-09-12 (1.17 to 37.98 hours 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 consistent within the GBM 90% localisation region. We find no evidence of this 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). However, we caution that the last available observation of the field was taken by ATLAS nine days before the GRB trigger.

Name | RA(J2000) | Dec(J2000) | Filter | Mag(AB) | t - trig(hrs)

GOTO23akf | 03:50:00.51 | -29:49:30.66 | L | 19.22 +/- 0.10 | 1.17 

This source is seen to decay as a power-law with an index of 0.61 +/- 0.07 across 7 epochs of observations. No object is present at this position in the Legacy Survey (Dey et al. 2019).

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.

GOTO ( 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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