TITLE: GCN CIRCULAR NUMBER: 43931 SUBJECT: EP260306a: COLIBRÍ optical observations of the fading and red counterpart DATE: 26/03/06 04:38:09 GMT FROM: globus@astro.unam.mx Noémie Globus (UNAM), Benjamin Schneider (LAM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), 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) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of EP260306a (Huang et al., GCN Circ. 43930) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-06 02:41 to 03:34 UTC (from 1.20 to 2.10 hours after the trigger) and obtained 23, 12 and 39 minutes of simultaneous exposure in the r, i, 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 PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. We detect the optical counterpart reported by Li et al. (GCN Circ. 43929) at preliminary magnitudes of: r = 23.27 +/- 0.25, z = 21.58 +/- 0.12. Comparing our observations with those reported by Li et al. (GCN Circ. 43929), the transient has faded, and we estimate a temporal decay index alpha ≈ 1. A preliminary fit to the SED suggests a high-z event, with a tentative photometric redshift between z=4 and z=5, but cannot rule out an intrinsically redder source at a lower redshift. We encourage spectroscopy. We notice that at the position of the optical counterpart, there is a faint galaxy with g = 24.91 +/- 0.28, r = 23.90 +/- 0.17, i = 23.54 +/- 0.24, z = 23.30 +/- 0.26, and photo-z = 0.72 +/- 0.27 in the Legacy Survey catalog (Dey et al., 2019). Furthermore, at a distance of 17 arcsec from the source, there is a nearby (photo-z = 0.06 +/- 0.01) bright galaxy with g = 18.04 +/- 0.01, r = 17.34 +/- 0.01, i = 17.02 +/- 0.01, and z = 16.76 +/- 0.01, with a chance coincidence probability Pcc~2%. Both of these galaxies may be unrelated if the event is located at high redshift. Additional observations in g, r, i, z bands and analysis are still ongoing. 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/43931. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...