TITLE: GCN CIRCULAR NUMBER: 43806 SUBJECT: EP260221a: COLIBRÍ optical counterpart candidate DATE: 26/02/22 17:22:21 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Antonio de Ugarte Postigo (LAM), Rosa L. Becerra (UNAM), Benjamin Schneider (LAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Fredd Sánchez Álvarez (UNAM) report: We reobserved the field of the EP260221a (Zhang et al., GCN Circ. 43791) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-22 05:55 to 10:54 UTC (from 18.20 to 23.20 hours after the trigger) and obtained 71 minutes of simultaneous exposure in the r and z filters. The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the Pan-STARRS-DR2 catalog, is in the AB system, and is not corrected for Galactic extinction. When we subtract the r-band image from our second epoch from the one from our first (Mandarakas et al., GCN Circ. 43793), we detect a residual source with a SNR of about 5 at: RA (J2000) = 11:55:01.22 = 178.7551 deg Dec (J2000) = -20:58:57.6 = -20.9827 deg The uncertainty on this position is 0.5 arcsec. The source is also seen in subtractions against PS1 images, but at a lower significance, and lies within the enhanced FXT uncertainty region (Liu et al., GCN Circ. 43802). We suggest it is the optical counterpart of the EP source. We notice that these coordinates are consistent with the position of a galaxy present in the Legacy Survey catalog (Dey et al., 2019) with magnitudes g = 23.68 +/- 0.17, r = 23.29 +/- 0.18, i = 22.41 +/- 0.12, and z = 21.90 +/- 0.15 and with a photo-z = 1.22 +/- 0.55. In the r band, the total magnitude of the galaxy and the candidate counterpart faded from 22.54 +/- 0.08 in our first epoch to 23.39 +/- 0.15 in our second epoch (a difference of 0.85 +/- 0.17) and in the z band it faded from 22.03 +/- 0.17 to 22.69 +/- 0.16 (a difference of 0.66 +/- 0.23). These fadings have significances of 5.0- and 2.8-sigma, respectively. The detection of this candidate in r as well as z is consistent with the photo-z of the coincident galaxy Therefore, we suggest that this galaxy might be the host. The non-detection of this source in deep imaging by Levan et al. (GCN Circ. 43805) suggests that it faded very quickly. This is perhaps consistent with temporal decay estimated from the two values reported in X-rays by (Liu et al., GCN Circ. 43802) alpha of approximately 2.2 and suggests the presence of an early jet break. 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/43806. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...