TITLE: GCN CIRCULAR NUMBER: 39458 SUBJECT: GOTO observations of LVK S250223dk/Swift-BAT RAVEN alert DATE: 25/02/24 16:54:52 GMT FROM: d.s.oneill@bham.ac.uk D. O’Neill, B. P. Gompertz, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, A. Kumar, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Pall'e 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) in response to the joint LVK/Swift-BAT RAVEN alert (LIGO, Virgo, KAGRA and Swift/BAT-GUANO teams, GCN 39443). Targeted observations of the combined 90% credible region (using the combined-ext.multiorder.fits,2 skymap) were performed by GOTO-South between 2025-02-24 09:41:31 UT (+21.67h) and 2025-02-24 13:18:31 (+25.28h) UT. 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 catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. In the 5 arcmin circle centred at RA = 85.341, Dec = -47.544, containing 52% of the integrated joint probability (LIGO, Virgo, KAGRA and Swift/BAT-GUANO teams, GCN 39443), no credible counterpart candidates are identified to a 3-sigma limiting magnitude of L > 21.1 mag. Furthermore, no candidate optical counterparts are identified within the <1 deg2 combined 90% LVK+Swift/BAT-GUANO credible region. In addition to the targeted response, serendipitous imaging from the all-sky survey was obtained for the 90% credible region defined by the NITRATES search, starting from 2025-02-23 12:03:34 UT (+0.04h). This resulted in a total coverage of 1621 deg^2 within the 90% localisation contour of Swift/BAT-GUANO, corresponding to ~41.6% of its total 2D localisation probability. No credible optical counterparts were identified within this region. 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/39458. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...