TITLE: GCN CIRCULAR NUMBER: 44875 SUBJECT: GRB 260607A / GOTO26fqf / AT2026ong: COLIBRÍ optical observations DATE: 26/06/08 18:24:05 GMT FROM: Massimiliano Lincetto at Aix Marseille Univ, CNRS/IN2P3, CPPM <lincetto@cppm.in2p3.fr> Massimiliano Lincetto (CPPM), Leonardo García García (UNAM), Jean-Grégoire Ducoin (CPPM), Benjamin Schneider (LAM), 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), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), 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) and Antonio de Ugarte Postigo (LAM) report: We imaged the candidate optical counterpart GOTO26fqf / AT2026ong (Gompertz et al., GCN Circ. 44864) of the Fermi GRB 260607A (Fermi GBM, GCN Circ. 44855) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-08 09:42 to 09:58 UTC (23 hours after the trigger) and obtained 840 seconds of simultaneous exposure in the r/z filters. The data were reduced and coadded with the COLIBRÍ ASU pipeline and analysed in STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. After subtracting template Pan-STARRS images, we detected the optical counterpart reported by Gompertz et al., GCN Circ. 44864, at preliminary magnitudes of: r = 19.72 +/- 0.01 z = 20.05 +/- 0.04 The measurement is consistent with a no-evolution scenario with reference to the GOTO measurement taken 14.7h after the trigger. The color r-z = -0.33 indicates a blue object, not consistent with typical GRB afterglows. 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/44875. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...