TITLE: GCN CIRCULAR NUMBER: 43486 SUBJECT: GRB 260120B: COLIBRÍ optical upper limits DATE: 26/01/21 03:39:32 GMT FROM: globus@astro.unam.mx Noémie Globus (UNAM), Antonio de Ugarte Postigo (LAM), 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), 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), and Benjamin Schneider (LAM): We imaged the field of the Swift GRB 260120B (Ferro at al., GCN Circ. 43479; Goad et al., GCN Circ. 43484), also detected by Fermi (Fermi GBM team, GCN Circ. 43477) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-21 02:00 to 02:19 UTC (from 8.01 to 8.31 hours after the trigger) and obtained 16 minutes of simultaneous exposure in the r and z filters during evening twilight. The data were reduced and coadded 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 do not detect any source within the enhanced XRT error circle (Goad et al., GCN Circ 43484) down to the following 3-sigma limits: r > 22.87, z > 22.12. We do not detect the afterglow candidate reported by Wu et al (GCN Circ. 43483). This is consistent with the source having faded below our detection limit by the time of our observation, and indicates a temporal index steeper than -1.0. 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/43486. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...