TITLE: GCN CIRCULAR NUMBER: 40117 SUBJECT: GRB 250407A: COLIBRÍ/DDRAGO Detection of the Optical Counterpart DATE: 25/04/08 08:55:29 GMT FROM: J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr> Jean-Grégoire Ducoin (CPPM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), Rosa L. Becerra (U Roma), William H. Lee (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), 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), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of GRB 250407A (Fermi GBM team, GCN Circ. 40104; Preis et al., GCN Circ. 40105) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-08 UTC. We observed from 03:01 to 03:53 UTC (11.21 to 12.09 hours after the trigger) and obtained 20 minutes of exposure in the i filter. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction. At the position reported previously by SRG/ART-XC (Molkov et al. GCN Circ. 40110) and Swift/XRT (Evans et al., GCN Circ. 40113), we detect a source with a magnitude of i = 19.96 +/- 0.03, consistent with the value reported by Zheng et al. (GCN Circ. 40116). In this position, there is a previously identified source (ID: 152151107588088917) in the Pan-STARRS catalog with a PSF magnitude of i = 22.99 +/- 0.14. We propose that this is the host galaxy of GRB 250407A. Further observations are planned. We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir. View this GCN Circular online at https://gcn.nasa.gov/circulars/40117. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...