TITLE: GCN CIRCULAR NUMBER: 43584 SUBJECT: EP260131a: COLIBRÍ observations of slow optical fading DATE: 26/01/31 15:00:38 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Benjamin Schneider (LAM), Alan M. Watson (UNAM), Fredd Sánchez Álvarez (UNAM), Jean-Luc Atteia (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the EP260131a (Ding et al., GCN Circ. 43574; Wu et al, GCN CIrc 43583) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-31 04:40 to 11:52 UTC (from 1.14 to 8.35 hours after the trigger) and obtained, respectively, 127, 191, and 320 minutes of exposure in the g, r, and z filters. The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. The candidate optical counterpart reported by Li et al. (GCN Circ. 43577), Sánchez Álvarez et al (GCN Circ. 43578), Malesani et al. (GCN Circ 43581), and Schneider et al. (GCN Circ. 43582) is clearly detected in our data. It fades slowly from r ≈ 21.1 to r ≈ 21.4 and z ≈ 20.8 to z ≈ 21.2 during our observations with an apparent average temporal power-law index of -0.16 +/- 0.07. We observe that the g-r color is 0.14 +/- 0.06, consistent with the detection by Schneider et al. (GCN Circ. 43582) down to the atmospheric cutoff. We encourage further observations, especially over the next 24 hours. 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/43584. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...