GCN Circular 34398

K. Ackley; B. P. Gompertz; B. Godson; S. Belkin; D. O'Neill; A. Levan; T. Killestein; G. Ramsay; D. Malesani; R. Starling; M. J. Dyer; J. Lyman; K. Ulaczyk; F. Jiminez-Ibarra; A. Kumar; D. Steeghs; D. K. Galloway; V. Dhillon; P. O'Brien; 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 230812B (Page et al. GCN 34394
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, Scotton et al. GCN 34392
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, Lesage et al. GCN 34391
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). We covered the field of the X-ray (Swift, Page et al. GCN 34394
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) and optical (KAIT Zheng et al. GCN 34395
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, MASTER Lipunov et al. GCN 34396
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) candidate afterglow. The field was observed several times between 21:08:01 UT and 23:37:33 UT on 2023-08-12 (starting 2.16 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. We confirm the optical afterglow as reported by KAIT (Zheng et al. GCN 34395
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) with the first detection at 2.29 hours. Our observations show a clear decay as the source faded by approximately 0.76 magnitudes over 2.27 hours.

Obs Date | RA(J2000) | Dec(J2000) | Filter | Mag(AB)
2023-08-12 21:15:40 | 249.13 | 47.86 | L | 17.45 +/- 0.02

We note the presence of an underlying extended source at the KAIT localisation in PS1 imaging and deep HyperSuprimeCam imaging, and suggest it to be the host galaxy of GRB 230812B. 

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 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).