TITLE: GCN CIRCULAR NUMBER: 45264 SUBJECT: EP260731a: COLIBRÍ optical upper limit DATE: 26/07/31 07:24:07 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> Antonio de Ugarte Postigo (LAM), 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), Jean-Grégoire Ducoin (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), Benjamin Schneider (LAM), Alan M. Watson (UNAM) report: We imaged the field of EP260731a (Ni et al., GCN Circ. 45261) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-07-31 05:52:50 to 07:10:03 UTC (from 4.9739 to 6.2776 hours after the trigger) and obtained 39x60 s of simultaneous exposure in the z-band filter. Some further images obtained during this time window were lost due to passing clouds. The data were reduced, coadded and analysed with the COLIBRÍ ASU and CPPM pipelines. The photometry was calibrated using nearby stars from the PanSTARRS 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 (Ni et al., GCN Circ. 45261) down to the following 5-sigma limit: z > 20.9 This upper limit is consistent with the one reported by Liu et al. (GCN Circ. 45263). Further observations 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/45264. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...