TITLE: GCN CIRCULAR NUMBER: 43170 SUBJECT: EP251220a: COLIBRÍ confirmation of fading and photometric redshift upper limit of z < 3.4 DATE: 25/12/21 11:43:02 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 reobserved the field of 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 04:35 to 07:13 UTC (from 20.13 to 22:51 hours after the trigger) and obtained 8, 32, 8, 32, and 5 minutes of exposure, respectively, in the g, r, i, z, and y 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. Compared to our first epoch of observations (Sánchez Álvarez et al., GCN Circ. 43167), the afterglow has faded with a temporal index of -1.38 +/- 0.25. This confirms the association with the X-ray transient. We detect the afterglow in all filters. After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.037 mag (Schlafly & Finkbeiner 2011) and fitting a power-law with a SMC dust extinction model to the grizy-bands, we estimate a photometric redshift upper limit of z < 3.4. 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/43170. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...