TITLE: GCN CIRCULAR NUMBER: 44834 SUBJECT: GRB 260604C: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 26/06/05 04:19:38 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx> Rosa L. Becerra (UNAM), Sarah Antier (IJCLAB), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), 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), Antonio de Ugarte Postigo (LAM), and Sofia Bisero (CEA Saclay) report: We imaged the field of GRB 260604C (Fermi Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-05 03:41 to 03:51 UTC (from 7.34 to 7.52 hours after the trigger) and obtained 3, 3, 3, 3 and 6 minutes of exposure in the g, r, i, z and y filters respectively. The data were reduced, coadded and analysed with the ASU COLIBRÍ 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 detected the optical counterpart reported by Lipunov et al. (GCN Circ. 44827), at preliminary magnitudes of: g = 19.69 +/- 0.04 r = 19.56 +/- 0.03 i = 19.38 +/- 0.04 z = 19.21 +/- 0.06 y = 19.26 +/- 0.08 These values are consistent with those reported by MASTER (Lipunov et al., GCN Circ. 44827), LAST (Gotz et al., GCN Circ. 44828), Jinshan (Zhu et al., GCN Circ. 44832). Comparing to the observations with LAST at about 0.75 hours, we estimate a temporal decay index of about -1.2. Moreover, the detections in all filters and the values in the colours, suggest a low-redshift event. We also notice the presence of a red galaxy with g = 21.69 +/- 0.02, r = 20.15 +/- 0.01, z = 19.10 +/- 0.01 and a photo-z = 0.343 +/- 0.031 (DESI Legacy Survey catalog; Dey et al. 2019) at 18.9" from the optical counterpart position. This corresponds to a Pcc ≈ 0.16 and a projected offset of 95 kpc, which makes an association very unlikely. We also comment that the light curve and analysis reported by the Fermi/GBM Team (GCN Circ. 44822, 44831) suggests hard emission with a duration that cannot completely rule out a Type I burst. Further observations and analysis 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 Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/44834. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...