TITLE: GCN CIRCULAR NUMBER: 43140 SUBJECT: GRB 251215A: COLIBRÍ optical upper limit DATE: 25/12/16 12:53:38 GMT FROM: Stephane Basa at UAR Pytheas/OHP, LAM <stephane.basa@lam.fr> Stéphane Basa (UAR Pytheas), Leonardo García García (UNAM), Benjamin Schneider (LAM), Noémie Globus (UNAM), Jean-Luc Atteia (IRAP), William H. Lee (UNAM), Alan M. Watson (UNAM), 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), Ramandeep Gill (UNAM), Enrique Moreno Méndez (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM). We imaged the field of GRB 251215A detected by Fermi (Fermi GBM team, GCN Circ. 43104) and confirmed by Swift/BAT-GUANO (Ronchini et al., GCN Circ. 43114) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-15 at 11:27:37.35 to 12:06:41.62 UTC (from 7.10 to 7.76 hr after the Fermi trigger) and obtained 31 minutes of simultaneous exposure in the r and z filters. Observations were obtained at a relatively high airmass (~3.0), and we also noted that the Galactic extinction is significant at the Swift/BAT-GUANO localization (Av= ~2.8). The data were reduced, coadded and analyzed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the SkyMapper-DR4 catalog, is in the AB system, and was not corrected for Galactic extinction. In the stacked image, we do not detect any new source associated with the candidates reported by Swift-XRT (Osborne et al., GCN Circ. 43137) down to the following 5-sigma limit: r > 21.8 z > 20.5 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/43140. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...