GCN Circular 37459

Y. Julakanti, A. Kumar, D. O’Neill, B. P. Gompertz, M. Kennedy, B. Godson, K. Wiersema, R. Starling, K. Ackley; 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. Palle 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 Fermi/GBM GRB 240910A (trigger 747633649.214168/240910167; Fermi GBM Team, GCN 37441
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; Zhang et al. GCN 37445
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; Ripa et al. GCN 37450
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). Targeted observations were performed by GOTO South from 2024-09-10 13:26:31 UT to 2024-09-11 02:36:55 UT (respectively from +9.42 to +22.60 hours post trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm).


In total, 283.9 square degrees within the 90% contour were imaged, resulting in a coverage total of ~89.1% of the total 2D localisation probability. 224 images were taken across 13 unique pointings, with an average 5-sigma depth of 19.9 mag. 


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 catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.

A new optical source GOTO24fvl/AT2024vfp
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 (https://www.wis-tns.org/object/2024vfp) is identified within the GBM 90% localisation region at J2000 coordinates RA = 01:36:23.45; Dec = -00:12:17.86. The source was initially detected at L-band magnitude of 19.23 +/- 0.12, 9.43 hours post trigger, with further detections at 19.65 +/- 0.12 mag (10.56 hours) and 19.63 +/- 0.14 mag, 12.01 hours post trigger, consistent with a decay rate t^(-1.56 +/- 1.20). The source was not detected down to a 5-sigma depth of L = 19.41 mag, 3.09 days prior to the GRB trigger.

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).

The rate of evolution, coupled with the spatial and temporal coincidence of GOTO24fvl/AT2024vfp
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 with GRB 240910A, make it a plausible afterglow candidate, but we caution we cannot confidently associate the two sources based on the available observations. A Swift ToO to search for associated X-ray emission has been requested (Evans et al., GCN 37456
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).


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


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).