TITLE: GCN CIRCULAR NUMBER: 43980 SUBJECT: GRB 260310A / AT 2026fgk: COLIBRÍ optical observations DATE: 26/03/13 04:01:22 GMT FROM: enriquemm@astro.unam.mx Enrique Moreno Méndez (UNAM), Massimiliano Lincetto (CPPM), Damien Dornic (CPPM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), 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 AT 2026fgk (Lipunov et al., GCN Circ. 43954; Konno et al., GCN Circ. 43975; Hinds et al., GCN Circ. 43977; Lipunov et al., GCN Circ. 43978, Martin-Carrillo et al., GCN Circ. 43079), a possible counterpart of the Fermi GBM GRB 260310A (Fermi GBM Team, GCN Circ. 43951; Humburg & Meegan, GCN Circ. 43975) and also detected by AstroSat CZTI (Salunke et al., GCN Circ. 43958), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-13 02:37 to 03:06 UTC (from 69.67 to 70.15 hours after the GRB trigger) and obtained 8, 8, 8, 8, and 16 minutes, respectively, of exposure in the g, r, i, z, and y filters. The data were reduced, coadded, calibrated, and analysed with the 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. In the stacked images, we detect AT 2026fgk with preliminary PSF magnitudes of: g = 18.53 +/- 0.01 r = 18.15 +/- 0.01 i = 17.95 +/- 0.01 z = 17.74 +/- 0.01 y = 17.63 +/- 0.02 Our photometry is consistent with the reported by Martin-Carrillo (GCN Circ. 43979) and with the colours expected for a low-redshift GRB, supporting its association with GRB 260310A (Fermi GBM Team, GCN Circ. 43951). As was firstly noticed by the LAST team (Konno et al.; GCN Circ. 43974), this source is projected separation of 11.3 kpc from the center of a nearby galaxy with photo-z ≈ 0.11 (Legacy Survey DR10, Dey et al. 2019) and likely spectroscopic redshift z = 0.153 (Hinds et al., GCN Circ. 43977). This source has a probability of chance alignment with the nearby galaxy of <1%, strongly suggesting an association. 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/43980. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...