TITLE: GCN CIRCULAR NUMBER: 41951 SUBJECT: GRB 250920C: COLIBRÍ optical observations DATE: 25/09/23 11:33:46 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> Antonio de Ugarte Postigo (LAM), Diego López-Cámara (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Benjamin Schneider (LAM): We imaged the field of the Fermi/GBM GRB 250920C (Fermi GBM team, GCN Circ. 41903; Mailyan et al. GCN 41917), also detected by Swift/BAT (Gupta et al. GCN 41904; Gupta et al. GCN 41924) and SVOM/GRM (Wang et al. GCN 41929) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-09-23 08:01:47 to 10:19:49 UTC (from 2.69 to 2.75 days after the trigger) and obtained 64 minutes of simultaneous exposures in the r and z 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 Wortley et al. (GCN Circ. 41907), at preliminary magnitudes of: r = 22.70 +/- 0.14 z = 22.54 +/- 0.25 We note that the source is still significantly brighter than the candidate host galaxy identified by Wortley et al. (GCN Circ. 41907) which appears in the Legacy Survey with r ~ 25.1 mag, so our observations are still afterglow dominated. Further observations and analysis 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/41951. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...