TITLE: GCN CIRCULAR NUMBER: 39488 SUBJECT: EP250226a/GRB 250226A: COLIBRÍ/DDRAGO Detection of the Optical Counterpart DATE: 25/02/26 10:37:22 GMT FROM: Francesco <francesco.magnani.work@gmail.com> Francesco Magnani (CPPM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), Benjamin Schneider (LAM), Alan M. Watson (UNAM), Kin Ocelotl López (UNAM), William H. Lee (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Antonio de Ugarte Postigo (LAM) report: We observed the field of EP250226a (Jiang et al., GCN Circ. 39482), consistent with the likely long GRB 250226A detected by Fermi/GBM (Fermi GCN Circ. 39479), with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico. We observed from 2025-02-26 07:56:43 to 09:14:20 UTC (82 minutes to 164 minutes after the trigger) and obtained 50 minutes of exposure in the i filter. The data were coadded with the COLIBRÍ pipeline and analyzed in STDWeb/STDPipe (Karpov 2021), with photometric calibration against Pan-STARRS DR1 and image subtraction against Pan-STARRS DR2. Our photometry is in the AB system and is not corrected for Galactic extinction. In our stacked image we detect a source at the position of the optical candidate reported by An et. al. (GCN Circ. 39486) and Zhu et. al. (GCN Circ. 39487) with magnitude: i = 19.05 +/- 0.03 We encourage continued monitoring of this source. 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/39488. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...