TITLE: GCN CIRCULAR NUMBER: 43859 SUBJECT: GRB 260226A: COLIBRÍ optical upper limits DATE: 26/02/27 06:16:42 GMT FROM: Camila Angulo Valdez at UNAM <camiangulo@astro.unam.mx> Camila Angulo (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), 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 Fermi GRB 260226A (Fermi GBM Team, GCN Circ. 43840, Fermi LAT, GCN Circ. 43844) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-27 02:54:20 to 03:52:25 UTC (from 16.27 to 17.25 hours after the trigger) and obtained 45 minutes of simultaneous exposures in the r and z filters. The data were reduced and coadded with the 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 the stacked images, comparing to LS (Dey et al. 2019), we do not detect any new source at the LAT source position (Depalo et al., GCN Circ. 43850) down to the following 5-sigma limits: r > 22.86 z > 21.99 These upper limits are consistent with the ones reported by Lipunov et al. (GCN Circ. 43847, 43848), Konno et al. (GCN Circ. 43852), Saikia et al. (GCN Circ. 43853), and Strausbaugh et al. (GCN Circ. 43858). 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/43859. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...