TITLE: GCN CIRCULAR NUMBER: 44310 SUBJECT: EP260416a: COLIBRÍ optical counterpart candidate DATE: 26/04/16 09:22:19 GMT FROM: J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr> Jean-Grégoire Ducoin (CPPM), Enrique Moreno Méndez (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (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), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the EP260416a (Hu et al., GCN Circ. 44307) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-16 05:36:04 to 06:51:03 UTC (from 0.11 to 1.36 hours after the trigger) and obtained 60 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with both the COLIBRÍ and COLIBRÍ ASU pipeline and analysed with STDWeb/STDPipe (Karpov 2025). 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 r band, we detect a source revealed by image subtraction using Legacy Survey as template and consistent with the FXT 20 arcsec error circle (Hu et al., GCN Circ. 44307) at: RA(J2000) = 13:15:20.49 = 198.8354 degrees Dec(J2000) = +31:35:49.4 = 31.5971 degrees with an uncertainty of 0.5 arcsec. The preliminary magnitude derived for that source is: r = 23.4 +/- 0.2 z > 22.8 (3 sigma-limit) The position of the source is coincident with a galaxy in the Legacy Survey catalog (Dey et al. 2019) with a photometry of g = 24.01 +/- 0.17 r = 24.05 +/- 0.30 z = 23.35 +/- 0.34. In our observations, we find evidence for brighter emission in the r band relative to the catalog value, suggesting tentative excess emission relative to the cataloged galaxy flux. However, the non-detection in the z band, combined with the lack of clear evidence for variability, prevents a firm confirmation . 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/44310. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...