TITLE: GCN CIRCULAR NUMBER: 42368 SUBJECT: GRB 251017A: COLIBRÍ optical observations of a late plateau DATE: 25/10/19 08:30:13 GMT FROM: Dalya Akl at New York University Abu Dhabi <dka2010@nyu.edu> Dalya Akl (NYUAD), Margarita Pereyra (UNAM), Noémie Globus (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Sarah Antier (IJCLAB), 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), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the Swift GRB 251017A (Gupta et al., GCN Circ. 42322) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-19 01:57 to 05:53 UTC (from 41.48 to 45.22 hours after the trigger) and obtained 96 minutes of simultaneous exposure in the i and z filters. The data were 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. Compared to our observations on the previous night (Angulo et al., GCN Circ. 42348), we find the optical counterpart has faded by only Delta i = 0.06 +/- 0.04 We see similar behaviour in z. These results suggest that we may be observing a late plateau. Further observations are planned and encouraged. 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/42368. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...