TITLE: GCN CIRCULAR NUMBER: 44125 SUBJECT: GRB 260310A / AT 2026fgk: COLIBRÍ photometric evidence of the emerging supernova DATE: 26/03/27 08:14:57 GMT FROM: enriquemm@astro.unam.mx Marion Guelfand (CPPM), Enrique Moreno Méndez (UNAM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (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), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Benjamin Schneider (LAM) and report: We imaged the field of the optical counterpart AT 2026fgk (O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-27 02:46 to 03:59 UTC (from 16.90 to 16.95 days after the trigger) and obtained 1080 seconds in g, r, and i filters, and 3240 seconds of exposure in the z filter. The data were reduced, coadded and analysed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, it is in the AB system, and it is not corrected for Galactic extinction. We have obtained spectral energy distributions at multiple epochs prior to T+12 days, all of which are well described by a single power law with a spectral index β ≈ −1. However, tonight we observe a significant increase in the g−r and g-i colours and a decrease in the i−z colour indicating the emergence of a thermal component in the spectral energy distribution. This behaviour suggests the onset of a supernova, in agreement with the report by de Ugarte Postigo et al. (GCN Circ. 44124) Further observations and analysis are ongoing. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. 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/44125. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...