TITLE: GCN CIRCULAR NUMBER: 39162 SUBJECT: GRB 250205A: COLIBRÍ/DDRAGO Optical Afterglow Detection DATE: 25/02/06 05:06:58 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (Università degli Studi di Roma Tor Vergata), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Jean-Grégoire Ducoin (CPPM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), and Benjamin Schneider (LAM) report: We imaged the field of the SVOM/ECLAIRs GRB 250205A (Saccardi et al., GCN Circ. 39154) with the DDRAGO wide-field camera on the COLIBRÍ (SVOM/F-GFT) telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico. We observed from 2025-02-06 03:01 to 04:48 UTC, with a midpoint of 6.34 hours after the event, and obtained 3840 seconds of exposure in the r filter in good weather conditions. The data were reduced and stacked using custom software and then calibrated against the PS1 catalog and analysed using STDPipe (Karpov 2021). We detect the afterglow (Gompertz et al., GCN Circ. 39156, and Schneider & Adami, GCN Circ. 39157) with r = 22.89 +/- 0.11 This magnitude is not corrected for Galactic extinction. Our measurement is significantly fainter than the earlier r magnitudes reported by Gompertz et al. (GCN Circ. 39156), Schneider & Adami (GCN Circ. 39157), Palmerio et al. (GCN Circ. 39159), and de Ugarte Postigo et al. (GCN Circ. 39160), confirming that this is the afterglow. Compared to the magnitude reported by de Ugarte Postigo et al., our measurement implies a temporal index of -0.80 +/- 0.08 between 1.76 and 6.34 hours. 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/39162. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...