TITLE: GCN CIRCULAR NUMBER: 43518 SUBJECT: GRB 260125A: NOT optical observations on GOTO26acv / AT 2026bjf DATE: 26/01/26 14:19:11 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), B. P. Gompertz (Birmingham), G. Corcoran (UCD), L. Izzo (INAF/OAC and DARK/NBI), A. Martin-Carrillo (UCD), S. Bijavara Seshashayana (NOT), report on behalf of a larger collaboration: We observed the optical candidate GOTO26acv / AT 2026bjf (Gompertz et al., GCN 43514) of the Fermi GRB 260125A (Fermi GBM team, GCN 43510) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out using the SDSS griz filters, and started on 2026 Jan 25.995 UT (16.27 hr after the Fermi/GBM trigger). Visual inspection reveals no obvious point source at the coordinates of GOTO26acv / AT 2026bjf, on top of the galaxy PGC 139241 (LEDA 139241), in any of the observed bands, though we note that the bright and complex morphology of the galaxy makes a clear identification tough. Image subtraction against the Legacy survey archival images yields an upper limit r > 22.75 (AB) at a mean time of 16.47 hr after trigger (5x150 s exposure time). This value is calibrated against the Pan-STARRS catalog and is not corrected for Galactic extinction. Our limit is notionally deeper than the extrapolation of the power-law decay reported by Gompertz et al. (GCN 43514), even after accounting for a GOTO/ALFOSC color term and the quoted decay index uncertainty. View this GCN Circular online at https://gcn.nasa.gov/circulars/43518. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...