TITLE: GCN CIRCULAR NUMBER: 37566 SUBJECT: EP240919a: GOTO optical upper limits DATE: 24/09/20 09:09:33 GMT FROM: Amit Kumar at University of Warwick, UK <amitkundu515@gmail.com> A. Kumar, B. P. Gompertz, M. Kennedy, D. O'Neill, G. Ramsay, S. Belkin, D. Steeghs, J. Lyman, R. Starling, K. Ackley, M. J. Dyer, K. Ulaczyk, F. Jimenez-Ibarra, 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; Dyer et al. 2024) in response to fast X-ray transient EP240919a (Liang et al. GCN 37561; Liang et al. GCN 37563). Targeted observations of the FXT localisation were performed by GOTO South (Siding Spring Observatory) at 2024-09-19 15:11:11 UT (23.5 minutes after the X-ray detection). The 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 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 catalogues. Human vetting was carried out in real time on any candidates that passed the above checks. No candidate optical counterparts are identified within FXT localisation (Liang et al. GCN 37561) up to a 3-sigma limiting L-band magnitudes of >19.3. This field was also covered by GOTO South as part of its routine all-sky survey 44.7 minutes pre-detection (at 2024-09-19 14:03:01 UT), and no new transient sources were seen in a 4x45s stacked image up to a 3-sigma limiting magnitude of >19.4 (L-band) compared to previous template images. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/37566. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...