TITLE: GCN CIRCULAR NUMBER: 41963 SUBJECT: GRB 250924A: COLIBRÍ optical observations DATE: 25/09/24 12:35:04 GMT FROM: Benjamin Schneider at MIT <bschn@mit.edu> Benjamin Schneider (LAM), Camila Angulo (UNAM), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (U Roma), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM) and Ny Avo Rakotondrainibe (LAM): We imaged the field of the Swift GRB 250924A (Siegel et al., GCN Circ. 41959) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-09-24 08:33:51 to 09:46:24 UTC (from 15 to 88 minutes after the trigger) and obtained images in the g, r, i, z and y 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 detect the optical counterpart reported by source Siegel et al., GCN Circ. 41959 and Becerra et al., GCN Circ. 41960 at: RA(J2000) = 3:27:29.71 = 51.8738 degrees Dec(J2000) = +74:39:33.7 = +74.6594 degrees with an uncertainty of 0.5 arcsec. The preliminary magnitudes derived for that source are: Start (UT) | mid T0+T (s) | Band | Exp | Mag | Error --------------------|---------------|------|------|------|------ 2025-09-24T09:01:29 | 2853 | g | 3x60 | 20.12 | 0.05 2025-09-24T08:33:51 | 1054 | r | 3x60 | 18.90 | 0.04 2025-09-24T09:03:50 | 2994 | i | 3x60 | 19.38 | 0.05 2025-09-24T08:33:51 | 1054 | z | 3x60 | 18.39 | 0.05 2025-09-24T09:02:40 | 3963 | y | 16x60 | 19.26 | 0.06 From subsequent r-band images obtained between 0.26 and 2.21 hr after the burst, we measure a temporal decay slope of alpha = 0.6 +/- 0.1. Further observations 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/41963. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...