TITLE: GCN CIRCULAR NUMBER: 43459 SUBJECT: EP 260116a: COLIBRÍ detection of a fading optical source DATE: 26/01/19 21:20:37 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Nathaniel R. Butler (ASU), Noémie Globus (UNAM), Alan M. Watson (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), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (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), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report: We observed the field of EP 260116a (Wang et al., GCN Circ. 43424) obtained with the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope on the nights of 2026 January 17, 18, and 19 UTC at median times of 30.8, 52.4, and 78.7 hours after the trigger and obtained 80, 96, and 128 minutes, respectively, of simultaneous exposure in the 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. We reported preliminary results from the first night in Magnani et al. (GCN Circ. 43436). In contrast to our initial analysis, a refined reprocessing and the subsequent follow up observations have revealed a fading optical source located within the EP/WXT error region (Wang et al., GCN Circ. 43424) at the coordinates RA = 07:35:14.14, Dec = +06:20:14.3 (J2000). At the last epoch, we measure preliminary magnitudes of r = 23.22 +/- 0.06, z = 22.81 +/- 0.14. Including these observations together with the two earlier epochs, we model the temporal evolution of the source flux with a power-law decay. We find Flux_r = t^(−0.33 +/- 0.12) and Flux_z = t^(-0.53 +/- 0.23). The source therefore exhibits a slow but consistent decay, which is statistically significant and supports its interpretation as the fading optical counterpart of EP 260116a. No existing source is visible at this position in the PS1 images. Further observations are planned. 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/43459. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...