GCN 44059: GRB 260310A / AT 2026fgk: COLIBRÍ evidence of a late rebrightening

2026-03-20T20:08:08.530Z | rev 0 | event: GRB 260310A
Rosa L. Becerra (UNAM), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Noémie Globus (UNAM), Enrique Moreno Méndez (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Antonio de Ugarte Postigo (LAM), and Benjamin Schneider (LAM) report:

We reimaged the field of AT 2026fgk, first reported by Hinds et al. (AstroNote 2026-65) and followed-up by a large number of facilities across multiple wavelengths (GCNs Circ. 43951, 43954, 43958, 43974, 43975, 43977, 43978, 43979, 43980, 43981, 43984, 43986, 43990, 43991, 43993, 43994, 43996, 44000, 44001, 44002, 44003, 44004, 44005, 44006, 44020, 44021, 44022, 44043, 44044, 44045, 44051, 44053, 44057, 44058), as a possible counterpart to 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 several epochs from 2026-03-20 03:35 to 11:21 UTC (from 9.94 to 10.27 days after the GRB trigger) 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. Compared with the photometry acquired on 2026-03-19, we observe a rebrightening in all three filters throughout the night, with a temporal index of alpha > 1.5. We encourage extended follow-up observations.

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.