TITLE: GCN CIRCULAR NUMBER: 43167 SUBJECT: EP 251220a: COLIBRÍ detection of the optical counterpart DATE: 25/12/21 05:53:22 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Fredd Sánchez Álvarez (UNAM), Ny Avo Rakotondrainibe (LAM), Leonardo García García (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), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Jean-Grégoire Ducoin (CPPM), 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 fast X-ray transient EP251220a (Liu et al., GCN Circ. 43162) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-21 04:08 UTC to 04:35 UTC (from 19.70 to 20.13 hr after the trigger) and obtained 18 minutes of simultaneous exposure in the r and z filters. The data were reduced, coadded and analyzed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and was not corrected for Galactic extinction. In the stacked image, we detect an uncataloged source at the FXT uncertainty region (Liu et al., GCN Circ. 43162) at RA (J2000) = 01:09:16.60 = 17.31915 deg Dec (J2000) = +19:05:32.3 = 19.09231 deg with an uncertainty of about 0.5 arcsec and with preliminary magnitudes of r = 21.15 +/- 0.02 z = 20.72 +/- 0.04 We suggest that this is the counterpart of the fast X-ray transient. Further observations will be necessary to detect fading. 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/43167. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...