TITLE: GCN CIRCULAR NUMBER: 42168 SUBJECT: GRB 251007B: COLIBRÍ optical upper limit DATE: 25/10/08 15:32:24 GMT FROM: Francesco at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com> Francesco Magnani (CPPM), Camila Angulo (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), 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), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), , Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We imaged the field of the Swift/SVOM GRB 251007B (Melandri et al., GCN Circ. 42147, Webb et al., GCN Circ. 42148) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-08 10:34 to 12:32 UTC (from 14.9 to 16.8 hours after the trigger) and obtained 80 minutes of simultaneous exposure in the i and z filters. The data were reduced and coadded with the COLIBRÍ pipeline and the tenue software, 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. In the stacked image, we do not detect a source at the SVOM/VT source position (Li et al., GCN Circ. 42163) down to the following 3-sigma limits: i > 23.3 z > 22.7 These upper limits are consistent with the ones reported by Wu et al. (GCN Circ. 42150). Our image is not deep enough to confirm fading of the source reported by Li et al. (GCN Circ. 42163). 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/42168. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...