TITLE: GCN CIRCULAR NUMBER: 39453 SUBJECT: EP250223a: COLIBRÍ/DDRAGO Detection of the Optical Counterpart and Slow Fading DATE: 25/02/24 13:00:21 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Jean-Grégoire Ducoin (CPPM), Francesco Magnani (CPPM), Benjamin Schneider (LAM), Alan M. Watson (UNAM), Kin Ocelotl López (UNAM), Stéphane Basa (UAR Pytheas), 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 imaged the field of EP250223a (Lian et al., GCN Circ. 39429), detected also by Swift/XRT (Kennea et al., GCN Circ. 39437), 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 2025-02-24 04:59 to 07:19 UTC (13.9 to 16.3 hours after the trigger) and obtained 96 minutes of exposure in the r 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 a stacked image of our first 50 minutes of exposure, we detect a source at the position of the optical afterglow candidate reported by Hauptmann et al. (GCN Circ. 39436) and many other groups (GCN Circs. 39439, 39440, 39441, 39445, 39447, and 39449) with magnitude: r = 19.87 +/- 0.01 Comparing our r magnitude to the similar ones reported by Hauptmann et al. (GCN Circ. 39436) and Pérez-Fournon et al. (GCN Circ. 39440), we see no more than slow fading between about 6 and 15 hours. 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/39453. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...