TITLE: GCN CIRCULAR NUMBER: 44567 SUBJECT: GRB 260511B: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 26/05/12 04:22:36 GMT FROM: William H. Lee at UNAM <wlee@astro.unam.mx> William H. Lee (UNAM), Dalya Akl (NYUAD), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), 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) and Antonio de Ugarte Postigo (LAM), L. Zhang (IHEP), and D. Kong (GXU) report: We imaged the field of the SVOM/ECLAIRs and Fermi/GBM GRB 260511B (Fermi GBM Team, GCN Circ. 44529; Godet et al., GCN Circ. 44532) and also detected by AstroSat (Arya et al. GCN Circ. 44560) and NuSTAR (Waratkar et al. GCN Circ. 44556) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-12 03:24 to 03:43 UTC (from 16.35 to 16.67 hours after the trigger) and obtained 16 minutes of simultaneous exposure in r and z filters. 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. At the position of the optical counterpart reported by SVOM/C-GFT (Wu et al. GCN Circ. 44547), MITSuME (Sasada et al. GCN Circ. 44549), TRT (Jiang et al. GCN Circ. 44551), SVOM/VT (Wu et al. GCN Circ. 44552), we detect a source with preliminary magnitudes of: r = 19.99 +/- 0.01, z = 19.67 +/- 0.02. Compared with the optical observations reported by de Ugarte Postigo (GCN Circ. 44565), we estimate an optical decay temporal index of alpha ≈ -1.3 for the source. 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/44567. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...