TITLE: GCN CIRCULAR NUMBER: 44738 SUBJECT: GRB 260429B / EP260429a: COLIBRÍ optical counterpart candidate DATE: 26/05/29 15:17:32 GMT FROM: F. Fortin at IRAP <ffortin.sci.edu@gmail.com> Francis Fortin (IRAP), 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), Jean-Grégoire Ducoin (CPPM), 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 reimaged the field of GRB 260429B / EP260429a (Wang et al., GCN Circ. 44426, Arya et al., GCN Circ. 44443) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-27 06:34 to 08:42 UTC (from 27.7 to 27.8 days after the trigger) and obtained 1.57 hours of exposure in the r and z filters. The data were reduced and coadded with the ASU COLIBRÍ 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. By comparing these observations with our first epoch reported by Fortin et al. (GCN Circ. 44434), we found that an uncatalogued optical source located at: RA (J2000) = 240.58812 (16:02:21.15) Dec (J2000) = -9.60116 (-09:36:04.2) and with an uncertainty of 0.5”, is no longer detected in this second epoch down to the following (3-sigma) limits: r > 23.90 z > 23.07 In our first epoch, for this source, we measured: r = 21.95 +/- 0.05 z = 21.08 +/- 0.04 Given the fading nature of the source, the spatial coincidence with the source EPF J160221.2-093607 reported in by the EP team (Wang et al., GCN Circ. 44445), and the absence of any known minor planet at this position, we suggest this is the optical counterpart of GRB 260429B / EP260429a. 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/44738. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...