TITLE: GCN CIRCULAR NUMBER: 41995 SUBJECT: GRB 250925A: COLIBRÍ optical photometry of the afterglow DATE: 25/09/26 05:52:29 GMT FROM: enriquemm@astro.unam.mx Enrique Moreno Méndez (UNAM), N. A. Rakotondrainibe (LAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM): We imaged the field of the Swift GRB 250925A (Dichiara et al., GCN Circ. 41985) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-09-26 02:45 to 04:30 UTC (from 9.7 to 11.4 hours after the trigger) and obtained 32/32/64 minutes of exposure in the r/i/z filters. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. We detected the optical source reported by Corcoran et al. (GCN Circ. 41990) at preliminary magnitudes of: r = 23.07 +/- 0.18 i = 21.62 +/- 0.07 z = 21.02 +/- 0.06 The source has faded, confirming that it is the afterglow. Compared to the early observations reported by Corcoran et al., we derive a temporal index of about -0.9 in z. The very red r-i color is suggestive of a GRB at a moderately high redshift with a break in r. Further observations are ongoing. 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/41995. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...