GCN 45204: EP260723a: COLIBRÍ optical observations

Stéphane Basa (UAR Pytheas), Enrique Moreno Méndez (UNAM), Jean-Grégoire Ducoin (CPPM), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (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), 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), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM)

We imaged the field of the EP260723a (Zhang et al., GCN 45201) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-07-24T07:43:35.559 to 2026-07-24T09:27:41.815 UTC (from 12.96 to 14.68 hours after the trigger) and obtained 64 minutes of simultaneous exposure in the r and z filters.

The data were reduced, coadded, and analysed with the 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.

In the stacked images, we detect the optical counterpart reported by Eyles-Ferris et al. (GCN Circ. 45202) and He et al. (GCN Circ. 45203), with the following preliminary photometry:
  r = 22.25 +/- 0.09
  z = 21.57 +/- 0.14

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.