TITLE: GCN CIRCULAR NUMBER: 43426 SUBJECT: GRB 260111A: COLIBRÍ fading of the optical counterpart candidate DATE: 26/01/16 17:02:10 GMT FROM: Benjamin Schneider at MIT <bschn@mit.edu> Benjamin Schneider (LAM), Enrique Moreno Méndez (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM): We imaged the field of the Fermi/Swift GRB 260111A (Fermi GBM Team, GCN 43370, GCN 43384; Ronchini et al., GCN 43377, Luo et al., GCN 43385, Pfeiffer et al., GCN 43397) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-16 10:52 to 13:22 UTC (from 4.77 to 4.87 days after the trigger) and obtained 89 and 75 minutes of 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. The optical afterglow candidate reported by Ducoin et al., GCN 43386, consistent with the XRT #2 source (Lanava et al., GCN 43388), is no longer detected in the images down to the following 3-sigma limit: r > 24.4 z > 23.0 Our measurements indicate fading, and we propose this source as the optical afterglow of GRB 260111A. 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/43426. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...