TITLE: GCN CIRCULAR NUMBER: 44791 SUBJECT: GRB 260603A : SVOM/COLIBRÍ optical counterpart candidate DATE: 26/06/03 06:54:17 GMT FROM: Marion G <marion.guelfand@gmail.com> Marion Guelfand (CPPM), Alan M. Watson (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), 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), Ruizhi Li (YNAO), and Ziming Wang (BNU) report: We imaged the field of the SVOM GRB 260603A (Zhao et al., GCN Circ. 44787) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-06-03 05:39:27 to 06:02:56 UTC (from 133 seconds to 27 minutes after the trigger) and obtained 1000 seconds of simultaneous exposure in the r/z filters. The data were reduced, coadded, and analyzed with the COLIBRI ASU pipeline. The photometry was calibrated using nearby stars from the Pan-STARRS-DR2 catalog, is in the AB system, and is not corrected for Galactic extinction. We detect an uncatalogued source consistent with the ECLAIRs 5.42 arcmin error circle (Zhao et al., GCN Circ. 44787) at: RA (J2000) = 15:32:34.55 = 233.14397 degrees Dec (J2000) = +40:01:33.3 = 40.02591 degrees The preliminary magnitudes derived for this source are: r = 19.78 +/- 0.01 z = 19.27 +/- 0.01 The source is observed to brighten and then fade. We suggest this is the counterpart of the GRB. Further observations are ongoing. 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 Observa View this GCN Circular online at https://gcn.nasa.gov/circulars/44791. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...