TITLE: GCN CIRCULAR NUMBER: 43168 SUBJECT: GRB 251221A: COLIBRÍ detection of the optical counterpart DATE: 25/12/21 06:43:18 GMT FROM: Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe@gmail.com> Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (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), Enrique Moreno Méndez (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM): We imaged the field of the Fermi/Swift GRB 251221A (Fermi GBM team, GCN Circ. 43164; Ambrosi et al., GCN Circ. 43166) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-21 05:25:46 to 06:08 UTC (from 75 seconds to 43 minutes after the trigger) and obtained 1690 seconds of simultaneous exposure in the r and z filters. The data were reduced and coadded 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. We detected an uncatalogued, fading source at RA (J2000) = 06:45:42.73 = 101.42804 deg Dec (J2000) = +00:43:54.8 = 0.73189 deg with an uncertainty of 0.5 arcsec and preliminary magnitudes of: r > 24.2 (3-sigma) z = 22.26 +/- 0.18 The XRT uncertainty region (Ambrosi et al., GCN Circ. 43166) has a 90% confidence radius of 1.8 arcsec, and this source is 2.3 arcsec from the center. We suggest therefore that this is the optical counterpart of the GRB. The lack of detection in r is consistent with the high Galactic extinction in this direction of A_V = 3.1 mag (Schlafly et al. 2011). 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/43168. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...