TITLE: GCN CIRCULAR NUMBER: 43820 SUBJECT: GRB 260223A: COLIBRÍ optical afterglow decay confirmation DATE: 26/02/24 09:24:20 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> Antonio de Ugarte Postigo (LAM), Fredd Sánchez Álvarez (UNAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Edilberto Aguilar-Ruiz (UNAM), Dalya Akl (NYUAD), Camila Angulo (UNAM), Sarah Antier (IJCLAB), 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), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Benjamin Schneider (LAM) report: We continued to observe 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 in two epochs between 2026-02-24 02:23:16 and 07:15:39 UTC (between 22.20 and 27.07 hours after the trigger) obtaining images in the grizy filters on the first and rz on the second. The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR2, is in the AB system, and is not corrected for Galactic extinction. The optical counterpart reported by Becerra et al. (GCN Circ. 43811) and confirmed by our earlier observations (de Ugarte Postigo et al. GCN Circ 43812) has decayed after the early rebrightnening (de Ugarte Postigo et al. GCN Circ 43814). On our latest stack, we detect the afterglow of GRB 260223A at r = 20.93 +/- 0.07 mag. Between our last observation obtained yesterday and today’s observations, we measure a decay slope of alpha ~ -1.1 (where F_nu ~ t ^ alpha). This is consistent with the observations obtained by SVOM/VT a few hours before (Li et al. GCN Circ. 43816) and serves to confirm that this object is indeed the afterglow of GRB 260223A. 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/43820. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...