TITLE: GCN CIRCULAR NUMBER: 43599 SUBJECT: GRB 260131A/B (AT2026bwg): COLIBRÍ further optical observations DATE: 26/02/02 04:28:39 GMT FROM: Leonardo Enrique García García <lgarcia@astro.unam.mx> Leonardo García García (UNAM), Benjamin Schneider (LAM), Noémie Globus (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report: We observed the optical afterglow candidate AT2026bwg (Gompertz et al., GCN 43586) of GRB 260131A/B (Fermi/GBM Team, GCN 43579; Sugai et al., GCN 43580; Roberts et al., GCN 43585; Frederiks et al., GCN 43590; Luo et al., GCN 43591) with the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-01 02:10 to 03:07 UTC (from 43.52 to 44.48 hours after the trigger) in the g/r/i/y/z bands and obtained 13 minutes of exposure in the r band. The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. The optical counterpart (Gompertz et al., GCN 43586; Lee et al., GCN 43587; Saccardi et al., GCN 43595) is clearly detected in all filters, and has a preliminary magnitude of: r = 19.24 +/- 0.02. We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/43599. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...