TITLE: GCN CIRCULAR NUMBER: 42452 SUBJECT: GRB 251025B: SVOM/COLIBRÍ (FM-GFT) strong rebrightening of the optical counterpart DATE: 25/10/26 04:18:33 GMT FROM: Margarita Pereyra Talamantes at IA-UNAM Ensenada <mpereyra@astro.unam.mx> Margarita Pereyra (UNAM), Antonio de Ugarte Postigo (LAM), Benjamin Schneider (LAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Diego López-Cámara (UNAM), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and S. Hussein (IJCLab) report: We imaged the field of the SVOM GRB 251025B (Hussein et al., GCN Circ. 42437) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-10-26 02:54 to 03:17 (12.51 to 12.89 hours after the trigger) and obtained 6, 5, 5, and 16 minutes exposure in the the g, r, i, and z filters. The images were analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction. We detect the optical counterpart (Wu et al., GCN Circ. 42438; Gress et al., GCN Circ. 42439; Wu et al., GCN Circ. 42440; Beardmore et al., GCN Circ. 42445; Mohan et al., GCN Circ. 42447; Hernández Funget al., GCN Circ. 42450) with preliminary magnitudes of: g = 20.58 +/- 0.03 r = 20.20 +/- 0.03 i = 19.90 +/- 0.04 z = 19.79 +/- 0.04 We note that the afterglow appears to be brighter in r in our observations at about 12 hours than it was in the observations of Mohan et al. (GCN Circ. 42447) at about 2 hours, suggesting a very strong rebrightening. We further note a possible host galaxy candidate at the afterglow position in the Legacy Survey with r ≈ 23.3 and a photometric redshift if z ≈ 0.98 +/- 0.19. This is consistent with the detection in g. Further observations and analysis are ongoing. 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/42452. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...