TITLE: GCN CIRCULAR NUMBER: 42840 SUBJECT: GRB 251125B: COLIBRÍ observations of fading of the MASTER OT candidate DATE: 25/11/26 05:51:53 GMT FROM: globus@astro.unam.mx Noémie Globus (UNAM), Jean-Grégoire Ducoin (CPPM), Alan M. Watson (UNAM), Nathaniel R. Butler (ASU), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the MASTER OT J155558.56+581224.1 candidate afterglow (Gress et al., GCN Circ. 42838) of the Fermi GRB 251125B (Fermi GBM team., GCN Circ. 42836) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-11-26 01:36 to 02:39 UTC (median epoch 5.73 hours after the trigger) and obtained 48 minutes of simultaneous exposure in each of the r and z filters. 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 detect the optical candidate at preliminary magnitudes of: r = 20.46 +/- 0.02 z = 20.17 +/- 0.05 The source has faded considerably since the observation of Gress et al. (GCN Circ 42838), which confirms that it is likely to be the afterglow of the GRB. Further observations are planned. 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/42840. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...