TITLE: GCN CIRCULAR NUMBER: 41468 SUBJECT: LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits DATE: 25/08/21 16:29:36 GMT FROM: kendall.ackley@warwick.ac.uk K. Ackley, A. Kumar, D. O'Neill, M. J. Dyer, J. Lyman, K. Ulaczyk, S. Belkin, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, D. Pollacco, J. Casares Velazquez, T. Killestein 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 the LIGO/Virgo/KAGRA low-significance event S250818k (The LIGO-Virgo-KAGRA Collaboration, GCN 41437). As part of our strategy for LVK O4 follow-up, targeted observations of the event between 2025-08-18 01:22:29 UT (2.4 minutes post-trigger) and 2025-08-21 05:29:32 UT (76.2 hours post trigger) with GOTO-North and between 2025-08-18 08:34:23 UT (7.2 hours post-trigger) and 2025-08-19 09:29:09 UT (32.2 hours post-trigger) with GOTO-South. We covered a total of 521.4 deg2 (46.0%) of the localisation probability of the Bilby skymap (The LIGO-Virgo-KAGRA Collaboration, GCN 41440). Exposures were taken in L-band (400-700 nm passband) and the average 5 sigma depth was L = 19.95 AB magnitudes with GOTO-1 (North), L = 19.74 AB with GOTO-2 (North), L = 19.05 AB with GOTO-3 (South), and L = 19.70 AB with GOTO-4 (South). 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 significant counterpart sources that can be associated with S250818k have been identified. We do not detect emission of the candidate counterpart AT 2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) down to a 3-sigma limiting magnitude of L = 20.3 AB at 2025-08-18 20:53:41 UTC (~19.6 hours post-trigger). Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and 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/41468. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...