TITLE: GCN CIRCULAR NUMBER: 43572 SUBJECT: EP260128a / GRB 260128A: COLIBRÍ optical upper limit DATE: 26/01/30 15:46:49 GMT FROM: Jean-Luc Atteia at IRAP <jean-luc.atteia@irap.omp.eu> Jean-Luc Atteia (IRAP), Enrique Moreno Méndez (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), 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), Asuka Kuwata (UNAM), 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 X-ray transient EP260128a / GRB 260128a (Zhou et al., GCN Circ. 43549, Ravasio et al., GCN Circ. 43556) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-30 04:48 to 10:12 UTC (from 29.14 to 35.53 hours after the trigger) and obtained 238 minutes of simultaneous exposure in the i and z filters. The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ ASU 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 do not detect any new source at the Liverpool Telescope proposed candidate position (Eyles-Ferris et al., GCN Circ. 43553) nor at the NOT proposed candidate position (He et al., GCN Circ. 43555), down to the following 3-sigma limits: i > 23.6 z > 23.1 These upper limits are consistent with the one reported by Malesani et al. (GCN Circ. 43570). We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. 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/43572. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...