TITLE: GCN CIRCULAR NUMBER: 38505 SUBJECT: GRB 241209A: COLIBRÍ detection of the afterglow DATE: 24/12/09 22:12:33 GMT FROM: Damien Dornic <ddornic@km3net.de> J.-G. Ducoin (CPPM), Francesco Magnani (CPPM), Damien Dornic (CPPM), Alan M. Watson (UNAM), Margarita Pereyra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), S. Antier (OCA), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Francis Fortin (IRAP) and Simona Lombardo (LAM), report: We imaged the field of GRB 241209D detected by Swift/BAT and Fermi/GBM (GCN Circ. 38488, 38489, 38491) during the commissioning of the COLIBRÍ (SVOM/F-GFT) telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico. The observations started 21 minutes after the GRB trigger. We observed with the engineering test camera in a red filter that approximates SDSS r. The data were reduced using custom software and then analysed and calibrated against the PS1 catalog using the STDWeb service (Karpov et al. 2022). In 4260 seconds of exposure from 2024/12/09 11:20:50 (UT) to 12:59:15, we detect the optical counterpart with: r = 20.04 +/-0.05 (SNR ~21.6) This source is right next to a catalogued galaxy in PAN-STARRs with rmag of ~21.44. Given the significant difference in magnitude and the slight visible offset with the PAN-STARRs object position, we consider this to be the Afterglow of GRB 241209D. We warmly thank the COLIBRI engineering team 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/38505. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...