TITLE: GCN CIRCULAR NUMBER: 42129 SUBJECT: GRB 251005C: COLIBRÍ optical observations DATE: 25/10/06 18:58:06 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Francis Fortin (IRAP), Jean-Luc Atteia (IRAP), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), 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): We imaged the field of the Swift GRB 251005C (Page et al., GCN Circ. 42113) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-06 11:23 to 12:30 UTC (from 19.28 to 20.40 hours after the trigger) and obtained 38 minutes of simultaneous exposure in the i and z filters. The data were reduced and coadded with the tenue software and analyzed 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. At the position of the XRT source reported by Goad et al. (GCN Circ. 42116) and at the position of the VT optical source reported by Palmerio et al. (GCN Circ. 42123), we do not detect any sources above our 3-sigma limits of i > 23.8 z > 23.3 However, at the position of the VT source there are hints of the presence of a faint resolved object at lower significance. Our results are consistent with the X-shooter photometry reported by Yadav et al. (GCN Circ. 42127). We caution that the apparent fading in the i band with respect to the earlier VT_R detection is not conclusive, as the VT_R band above 650 nm includes both i and z, and our limit in z alone is not much deeper than the original detection. 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/42129. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...