TITLE: GCN CIRCULAR NUMBER: 37651 SUBJECT: EP240930a: GOTO optical upper limits DATE: 24/10/01 08:13:03 GMT FROM: Amit Kumar at Royal Holloway - U. of London/U. of Warwick, UK amitkundu515@gmail.com
A. Kumar, B. P. Gompertz, G. Ramsay, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, D. O'Neill, 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) covering the EP-WXT detected fast X-ray transient (Tian et al. GCN 37648). The WXT localisation was covered by GOTO North on 2024-09-30 from 20:09:58 to 23:19:50 UT (2.869 to 6.033 hours post trigger) during its triggered response to GRB 240930B (Fermi GBM Team, GCN 37640). The observations consisted of 4x90s exposures in the GOTO L-band (400-700 nm), distributed across 4 epochs.
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.
No candidate optical counterpart is detected in the WXT localisation region down to a 5-sigma L-band limit of 19.8–20.9 (AB) across 4 epochs. The first observation, at 2.869 hours post-trigger, reaches a 5-sigma depth of 20.9 mag.
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).
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