TITLE: GCN CIRCULAR NUMBER: 44988 SUBJECT: EP260618a: COLIBRÍ optical counterpart candidate DATE: 26/06/19 05:09:23 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx> Rosa L. Becerra (UNAM), 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), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report: We imaged the field of the EP260618a (Yang et al., GCN Circ. 44978), possibly associated with GRB 260618A (Fermi GBM team GCN Circ. 44973), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-19 03:44 to 04:55 UTC (from 19.15 to 20.34 hours after the trigger) and obtained 55 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the ASU COLIBRÍ 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, which is particularly significant in this direction (A_r = 1.1 mag; Schlafly & Finkbeiner 2011) due to the proximity of the field to the Galactic plane. We detect an uncatalogued source, absent in Pan-STARRS template images and and located within the WXT error region (Yang et al., GCN Circ. 44978) at: RA(J2000) = 18:26:43.13 = 276.6797 degrees, Dec(J2000) = +23:50:37.8 = 23.8438 degrees, with an uncertainty of 0.5 arcsec. The preliminary magnitudes derived for this source are: r = 21.90 +/- 0.04, z = 21.49 +/- 0.08. We therefore suggest that this source is the optical counterpart of EP260618a. Further observations and analysis are ongoing. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. 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/44988. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...