TITLE: GCN CIRCULAR NUMBER: 40689 SUBJECT: GRB 250610B: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 25/06/11 13:50:37 GMT FROM: Benjamin Schneider at MIT <bschn@mit.edu> Benjamin Schneider (LAM), Diego López-Cámara (UNAM), Rosa L. Becerra (U Roma), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Antonio de Ugarte Postigo (LAM), A. Saccardi (CEA), and J. Rodriguez (CEA): We imaged the field of the SVOM GRB 250610B (Saccardi et al., GCN Circ. 40671) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-06-11 04:35 to 06:40 UTC (from 12.0 to 14.1 hours after the trigger) and obtained 32 minutes of exposure in each of the g, r and i 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 Schneider et al., GCN Circ. 40676; Gompertz et al., GCN Circ. 40677; Li et al., GCN Circ. 40678; Bochenek et al., GCN Circ. 40684, and Qiu et al., GCN Circ. 40685 at preliminary magnitudes of: g = 21.83 +/- 0.14 r = 21.56 +/- 0.10 i = 21.49 +/- 0.09 Our detections are consistent with the shallow decay reported by the SVOM/VT team (Qiu et al., GCN Circ. 40685) and imply a temporal decay index of ~0.6. Further observations 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/40689. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...