TITLE: GCN CIRCULAR NUMBER: 38317 SUBJECT: LIGO/Virgo/KAGRA S241125n: COLIBRÍ Upper Limits on a Optical Counterpart at the BAT Position DATE: 24/11/26 06:20:40 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), S. Antier (OCA), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), D. Akl (AUS), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), J.-G. Ducoin (CPPM), Simona Lombardo (LAM), Francesco Magnani (CPPM), and Margarita Pereyra (UNAM) report: We imaged the field of the Swift/BAT-GUANO candidate counterpart (DeLaunay et al., GCN Circ. 38308) of the GW compact binary merger candidate S241125n (LIGO/Virgo/KAGRA Collaborations, GCN Circ. 38305) during the commissioning of the COLIBRÍ (SVOM/F-GFT) telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico. We observed with the engineering test camera in a red filter that approximates SDSS r from 2024-11-26 02:08 to 05:49:58 UTC (25.1 to 28.8 hours after the event) and obtained 188 minutes of exposure on a field about 12 arcmin to a side centered on the BAT position given by DeLaunay et al. (GCN Circ. 38308). The data were reduced using custom software and then analysed and calibrated against the PS1 catalog using the STDWeb service. No clear candidate is identified to a 5-sigma upper limit of r > 23.0 This limit is below the catalog limit, so definite statements on the absence of an optical counterpart will require a second epoch of similar depth. Further observations are planned. We warmly thank the COLIBRÍ engineering team and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir. We warmly thank the GRANDMA IJCLAB team and S. Karpov for the access of the STDWeb service for STDPipe. View this GCN Circular online at https://gcn.nasa.gov/circulars/38317. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...