TITLE: GCN CIRCULAR NUMBER: 39197 SUBJECT: LIGO/Virgo/KAGRA S250206dm: GOTO observations of optical candidates AT 2025azn and AT 2025azm DATE: 25/02/07 09:41:10 GMT FROM: kendall.ackley@warwick.ac.uk K. Ackley, S. Belkin, D. Steeghs, B. P. Gompertz, Y. Julakanti, M. Kennedy, B. Godson, G. Ramsay, D. O'Neill, M. Dyer, F. Jiménez-Ibarra, J. Lyman, K. Ulaczyk, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, L. Nuttall, E. Pallé, D. Pollacco, T. Killestein, and A. Kumar report on behalf of the GOTO collaboration: We are in the process of observing the LIGO/Virgo gravitational wave event S250206dm (GCN 39175; GCN 39178) with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022). Targeted observations with GOTO-N were performed beginning at Feb. 6 2025 21:31:11 UT, (+0.09h post trigger) and continued through to Feb. 7 2025 04:07:48 UT (+6.71h post trigger). GOTO covered the field of AT 2025azn (Hosseinzadeh et al, GCN 39191) at 22:25:45UT on 2025-02-06 (~1 hour after trigger, and 6 hours prior to the reported detection by Hosseinzadeh et al, GCN 39191) as part of the follow-up campaign for S250206dm. The observation consisted of a 4x90s exposure set in the GOTO L-band (400-700 nm). Analysis at the location of the potential optical transient does not show evidence of the source to a 3-sigma limiting AB magnitude of L > 19.8. Our previous observation of the field of AT 2025azm (Hosseinzadeh et al, GCN 39191) occurred on 2025-01-27. We do not see any evidence of excess flux down to a 3-sigma limiting AB magnitude of L > 19.0. Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template 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. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. Further observations are scheduled during the coming nights. 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/39197. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...