TITLE: GCN CIRCULAR NUMBER: 44657 SUBJECT: GRB 260516D: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 26/05/17 06:24:29 GMT FROM: Leonardo Enrique García García <lgarcia@astro.unam.mx> Leonardo García García (UNAM), Francis Fortin (IRAP), 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), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), 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), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM) and Andrea Saccardi (CEA) report: We imaged the field of the SVOM/ECLAIRs GRB 260516D (M. Brunet et al., GCN Circ. 44649) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-17 04:04 to 05:51 UTC (from to 7.48 to 9.25 hours after the trigger) and obtained 15 minutes of exposure in the r filter and 44 minutes of exposure in the z filter. The data were reduced, coadded, calibrated, and analysed 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. We detect the source reported by Li et al. (GCN Circ. 44654) and de Ugarte Postigo et al. (GCN Circ. 44656) with preliminary unsubtracted magnitudes of: r = 21.65 +/- 0.07, z = 21.66 +/- 0.16. Using the combined r’ and r- band photometry from GCNs 44655 and 44656 together with our COLIBRI observations, we estimate the optical afterglow temporal decay index to be approximately alpha ≈ 1.4. 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/44657. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...