TITLE: GCN CIRCULAR NUMBER: 45257 SUBJECT: EP260728a: COLIBRÍ optical upper limit DATE: 26/07/30 10:16:03 GMT FROM: J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr> Jean-Grégoire Ducoin (CPPM), Fredd Sánchez Álvarez (UNAM), 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), 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), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Alan M. Watson (UNAM) report: We imaged the field of the EP260728a (Yang et al., GCN Circ. 45248) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. 2026-07-30T05:58:16 to 2026-07-30T07:24:22 UTC (from 37.2 to 38.6 hours after the trigger) and obtained 64 minutes of exposure in the z filters. The data were reduced, coadded and analysed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the SkyMapper DR4 catalog, is in the AB system, and is not corrected for Galactic extinction. In the stacked image, we do not detect any new source at the FXT source position (Yang et al., GCN Circ. 42250) down to the following 3-sigma limit: z > 21.5 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/45257. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...