TITLE: GCN CIRCULAR NUMBER: 42842 SUBJECT: GRB 251125B: COLIBRÍ further observations and photometric redshift upper limit of z < 3 DATE: 25/11/26 16:30:16 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Jean-Grégoire Ducoin (CPPM), Noémie Globus (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 (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), 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), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We reobserved 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 two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-11-26 11:44 to 13:00 UTC (from 15.34 to 16.60 hours after the trigger) and obtained 20 minutes of exposure in each of the g, r, and i filters and 60 minutes of simultaneous exposure in the z filter. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. Compared to our first epoch of observations (Globus et al., GCN Circ. 42840), the afterglow has continued to fade. After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.013 mag (Schlafly & Finkbeiner 2011) and fitting a power-law with a no-dust extinction model to the griz-bands, we estimate a photometric redshift upper limit of z < 3. We therefore encourage follow-up observations. 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/42842. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...