TITLE: GCN CIRCULAR NUMBER: 43057 SUBJECT: GRB 251208B: COLIBRÍ optical upper limits DATE: 25/12/09 18:32:15 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Francesco Magnani (CPPM), Diego López-Cámara (UNAM), 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), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We imaged the field of the Fermi/Swift GRB 251208B (Fermi GBM Team, GCN Circ. 43028; DeLaunay et al., GCN Circ. 43033; Bissaldi et al., GCN Circ. 43034) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-09 12:13 to 13:13 UTC (from 25.9 to 26.9 hours after the Fermi/GBM trigger) and obtained 45 minutes of simultaneous exposure in the r and z filters. The data were reduced, coadded, and analyzed with the 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. In our stacked images, at the position of the XRT afterglow candidate (Kennea et al., GCN Circ. 43050) we do not detect any new source to the following 3-sigma limits: r > 23.7 z > 22.7 Furthermore, we do not detect any new source at the LAT uncertainty region (Bissaldi et al., GCN Circ. 43034) down to the following 5-sigma limits: r > 23.1 z > 22.1 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/43057. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...