TITLE: GCN CIRCULAR NUMBER: 41525 SUBJECT: GRB 250823A: COLIBRÍ optical observations DATE: 25/08/24 12:59:39 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it> Alan M. Watson (UNAM), Damien Dornic (CPPM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), 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), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We imaged the field of GRB 250823A, detected by Swift/BAT (Siegel et al., GCN Circ. 41511) and Fermi/GBM (Fermi GBM team et al., GCN Circ. 41510) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-24 05:54 to 06:33 UTC (from 9.73 to 10.38 hours after the trigger) and obtained 32 minutes of exposure in the i filter. 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 first reported by Gompertz et al. (GCN Circ. 41512) and subsequently by Fernandez-Garcia et al. (GCN Circ. 41513), Strausbaugh & Cucchiara (GCN Circ. 41514), Malesani et al. (GCN Circ. 41520), Kuin et al. (GCN Circ. 41521), and Mal et al. (GCN Circ. 41523), at a preliminary magnitude of: i = 22.74 +/- 0.21 This measurement is contemporaneous and consistent with the value reported by SVOM/VT (Mal et al., GCN Circ. 41523). 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/41525. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...