TITLE: GCN CIRCULAR NUMBER: 43812 SUBJECT: GRB 260223A: COLIBRÍ optical observations DATE: 26/02/23 08:22:07 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> Antonio de Ugarte Postigo (LAM), Leonardo García García (UNAM), Rosa L. Becerra (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), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), 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), and Benjamin Schneider (LAM) report: We imaged the field of the Fermi GRB 260223A (Fermi GBM team et al., GCN Circ. 43808) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-23 06:16:02 to 06:26:30 UTC (from 2.081 to 2.256 hours after the trigger) and obtained 3x60 s of simultaneous exposure in the grizy filters. The data were reduced, coadded and analysed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR2, is in the AB system, and is not corrected for Galactic extinction (A_V = 0.68, Schlafly et al. 2011). We detected the optical counterpart reported by Becerra et al., GCN Circ. 43811, at preliminary magnitudes of: g = 19.44 +/- 0.06 r = 18.92 +/- 0.04 i = 18.52 +/- 0.05 z = 18.27 +/- 0.04 y = 18.23 +/- 0.07 Further observations are ongoing. 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/43812. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...