TITLE: GCN CIRCULAR NUMBER: 43544 SUBJECT: GRB 260127A: COLIBRÍ optical observations DATE: 26/01/28 17:12:02 GMT FROM: F. Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com> Francesco Magnani (CPPM), Diego López-Cámara (UNAM), Jean-Grégoire Ducoin (CPPM), 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), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), 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 imaged the field of the Swift/Fermi/GBM GRB 260127A (Fermi GBM Team, GCN Circ. 43528, Klingler et al., GCN Circ. 43529) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-01-28 09:48 to 13:24 UTC (from 16.0 to 19.5 hours after the trigger) and obtained 110 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ pipeline and analysed 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. At the location of the optical counterpart reported by Saccardi et al. (GCN Circ. 43530), consistent with the X-ray counterpart reported by Evans et al. (GCN Circ. 43532) and Osborne et al. (GCN Circ. 43535), we marginally detect a source at a preliminary magnitude of: r = 22.80 +/- 0.28 At this same position, we measure the following 3-sigma upper limit: z > 21.6 Our observations suggest the candidate has faded since the observations reported by Saccardi et al. (GCN Circ. 43530), Wu et al. (GCN Circ. 43537), and Hagio et al. (GCN CIrc. 43540). 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/43544. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...