TITLE: GCN CIRCULAR NUMBER: 44001 SUBJECT: GRB 260310A / AT 2026fgk: COLIBRÍ continuing optical observations DATE: 26/03/14 06:20:02 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Damien Dornic (CPPM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Camila Angulo (UNAM), 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), Massimiliano Lincetto (CPPM), 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) report: We again 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; Moren Méndez et al., GCN Circ 43980; de Ugarte Postiga et al., GCN Circ. 43984; Hsu et al., GCN Circ. 43986; Pursiainen et al., GCN Circ. 43990; Becerra et al., GCN Circ 43991; Li et al., GCn Circ. 43993; Stein et al., GCN Circ 43996; Martin-Carrillo et al., GCn Circ. 44000), 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 2026-03-14 02:46 to 05:07 UTC (from 93.82 to 96.17 hours after the GRB trigger) and obtained 15, 14, and 28 minutes, respectively, of exposure in the g, r, and z 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 continue to detect AT 2026fgk with a preliminary PSF magnitude of: r = 18.47 +/- 0.01 Compared to the magnitudes in our first epoch reported in Moreno Méndez et al. (GCN Circ. 43980), we estimate a temporal decay index of about -1.2, consistent with the normal decay observed in GRBs. Moreover, from the optical/IR magnitudes from our first epoch and the photometry reported with WINTER (Stein et al., GCN Circ. 43996) between T+71.5 and T+72.5 hours, we estimate a spectral index beta of about -1.1 also supporting a synchrotron spectrum and a likely origin in a GRB afterglow. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. 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/44001. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...