GCN Circular 37433

A. Kumar, D. O'Neill, B. P. Gompertz, J. Lyman, G. Ramsay, R. Starling, K. Ackley, M. J. Dyer, 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 observations with the Gravitational-wave Optical Transient Observer (GOTO, Steeghs et al. 2022) in response to GRB 240905B (Fermi GBM team, GCN 37389
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; Joshi et al., GCN 37391
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; Gupta et al., GCN 37418
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). Targeted observations of the GBM localisation were performed by GOTO North and South from 2024-09-05 09:29:19 UT to 2024-09-06 00:48:53 UT (respectively from +5.03 to +20.33 hours after trigger) distributed over four epochs. 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.

No candidate optical counterparts are identified within a circular region of radius 0.2 degrees centered on the Fermi/LAT localisation (Gupta et al., GCN 37418
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). The typical depth of the most constraining image is L > 20.7 mags (5-sigma), taken at 09:36:50 UT on 2024-09-05 (~5.1 hours after the GBM trigger). Forced photometry at the positions of 6 Swift/XRT detected uncatalogued X-ray sources (Brivio et al., GCN 37421
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) in this image similarly provides 5-sigma limiting L-band magnitudes of ~20.7 on each.

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and were 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).