TITLE: GCN CIRCULAR NUMBER: 43386 SUBJECT: GRB 260111A: COLIBRÍ optical afterglow candidate DATE: 26/01/13 07:27:49 GMT FROM: J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr> Jean-Grégoire Ducoin (CPPM), Diego López-Cámara (UNAM), Edilberto Aguilar-Ruiz (UNAM), Francesco Magnani (CPPM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Kin Ocelotl López (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We imaged the field of the Swift/Fermi GRB 260111A (Fermi GBM Team, GCN 43370, GCN 43384; Ronchini et al., GCN 43377) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-12 10:06 to 13:26 UTC (from 17.55 to 20.88 hours after the trigger) and obtained 145 minutes of simultaneous exposure in the r and z filters. The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ pipeline, and were also analyzed with STDWeb/STDPipe (Karpov 2025). 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 detect an uncatalogued source revealed by image subtraction using Legacy Survey as template, consistent with the BAT/GUANO 5 arcmin error circle (Ronchini et al., GCN Circ. 43377) and the XRT source #2 (Evans et al., GCN 43379) at : RA(J2000) = 15:18:20.830 = 229.58679 degrees Dec(J2000) = +51:58:25.82 = 51.97384 degrees with an uncertainty of 0.5 arcsec. The preliminary magnitude derived for that source is: r = 22.87 +/- 0.07 z = 22.33 +/- 0.14 We cannot confirm that the source is fading, however its proximity to XRT #2 suggests it could be the afterglow of GRB 260111A. This detection is very significantly fainter than the upper limit measured with GOTO (Gompertz et al., GCN 43381), and thus compatible with it. We encourage further follow-up of this candidate. 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/43386. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...