GCN Circular 38088

Y. Julakanti, G. Ramsay, B. P. Gompertz, K. Ackley, D. O'Neill, A. Kumar, R. Starling, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle and D. Pollacco report on behalf of the 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 GRB 241105A (Fermi GBM Team, GCN 38085
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 ). Targeted imaging covered the Fermi localisation region from 2024-11-05 16:19:10 UT, (+0.22h post trigger) to 2024-11-05 17:42:50 UT (+1.61h post trigger). In total, 277.9 square degrees within the 90% contour were imaged, resulting in a coverage total of ~84.3% of the total 2D localisation probability. The observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm). In total, 101 images were taken, across 10 unique pointings, with an average 5-sigma depth of 20.4 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 GOTO24ibf is identified within the GBM 90% localisation region. We find no evidence of the source down to 20.0 mag prior to the GRB trigger time in previous GOTO observations taken at 2024-11-05 10:48:54 UTC (t-t0 = -5.29h) or the ATLAS forced photometry server (Shingles et al. 2021). There is a faint ambiguous underlying source at the position of GOTO24ibf visible in DESI legacy survey images that we cannot definitively identify as a star or galaxy. Due to the onset of twilight at Siding Spring Observatory, only a single epoch of observations were obtained, preventing any assessment of the evolution of the source. We encourage further deeper follow-up observations.

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|  Internal name |  Time (UT) | t-t0 (h) | RA (J2000) |  Dec (J2000) |      Mag      |
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|  GOTO24ibf   | 2024-11-05 16:34:35  | 0.48 | 04:24:59.0 | -49:45:09.3 | 17.21 +/- 0.01 |

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).