TITLE: GCN CIRCULAR NUMBER: 42325 SUBJECT: GRB 251017A: COLIBRÍ Early Afterglow Detection DATE: 25/10/17 11:03:48 GMT FROM: Dalya Akl at American Uni. SHJ <dalyaakl.d@gmail.com> Sarah Antier (IJCLAB), Dalya Akl (NYUAD), Enrique Moreno Méndez (UNAM), Alan M. Watson (UNAM), Noémie Globus (UNAM), Ny Avo Rakotondrainibe (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the Swift GRB 251017A (Page et al., GCN Circ. 42322) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-17 08:41 to 09:15 UTC (from 1 min to 35 min after the trigger) and obtained 35 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with 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. We detected the optical counterpart reported by Page et al., GCN Circ. 4232, at the SVOM/UVOT, with preliminary magnitudes of our first images: r = 19.90 +/- 0.17 z = 17.94 +/- 0.09 We note that the r-z color is ~2 mag. Concurrent observations were taken by BOOTES (I. Perez-Garcia et al., 42323). Further observations in g/r/i/z/y are planned. We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. 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/42325. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...