TITLE: GCN CIRCULAR NUMBER: 40271 SUBJECT: EP250427A: COLIBRÍ Continuing Optical Observations DATE: 25/04/28 12:30:11 GMT FROM: Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com> Francesco Magnani (CPPM), Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM)report: We continued our observational campaign of EP250427a (Wang et al., GCN Circ. 40257) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-28 UTC. We observed from 2025-04-28 09:16 to 2025-04-28 09:26 UTC (29.6 to 29.8 hours after the trigger) and obtained 3 stacks in filter r, i, and g, of 3 minutes each. Our observations were performed under regular weather conditions. The data were co-added with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction. In the optical position (Perez-Garcia et al., GCN Circ. 40259; Liu et al., GCN Circ. 40260; Becerra et al., GCN Circ. 40261; Brivio et al., GCN Circ. 40264; Chornock et al., GCN Circ. 40265; Saccardi et al., GCN Circ. 40266; Pérez-Fournon et al., GCN Circ. 40267; and Xi et al., GCN Circ. 40269), we measured: r = 20.55 +/- 0.05 Compared to our first epoch (Becerra et al., GCN Circ. 40261), we estimate the fading nature of the OT with a temporal index ~1.2, consistent with the value reported by Swain et al. (GCN Circ. 40270). Further observations and analysis are ongoing. 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/40271. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...