GRB 260310A / SN 2026fgk: A Multi-Wavelength Study of a Nearby Underluminous Long GRB and SN with a Complex Afterglow
Ramandeep Gill, Rosa L Becerra, Antonio de Ugarte Postigo, Christina C Thöne, Alan M Watson, Noémie Globus, Jean-Grégoire Ducoin, Peter Veres, Stanley E Kurtz, Asuka Kuwata, Antonio Martín-Carrillo, Luca Izzo, Christophe Adami, Enrique Moreno Méndez, Nikos Mandarakas, Camila Angulo-Valdez, Stéphane Basa, William H Lee, Edilberto Aguilar-Ruiz, Dalya Akl, Margo F Aller, Miguel Ángel Aloy, Jie An, Sarah Antier, Jean-Luc Atteia, Nathaniel R Butler, Krittapas Chanchaiworawit, Philipe V De La Parra, Damien Dornic, Francis Fortin, Shaoyu Fu, Johan P U Fynbo, Lluis Galbany, Leonardo García-García, Stefan Geier, Marion Guelfand, Linbo He, Shuaiqing Jiang, Emeric Le Floc’h, Massimiliano Lincetto, Xing Liu, Gianluca Lombardi, Diego López-Cámara, Daniele Bjørn Malesani, Francesco Magnani, Kanthanakorn Noysena, Margarita Pereyra, Ny Avo Rakotondrainibe, Anthony C S Readhead, Delphine Russeil, Fredd Sánchez-Álvarez, Benjamin Schneider, Tirth D Surti, Nial R Tanvir, Samaporn Tinyanont, Dong Xu, Zipei ZhuAbstract
We present a comprehensive multi-wavelength study of GRB 260310A / SN 2026fgk, a nearby (z = 0.153), long-duration gamma-ray burst (GRB) with an exceptionally underluminous prompt γ-ray emission and a Comptonized spectrum. The burst occurred at the edge of a blue host galaxy at a projected distance of 15 kpc, which is one of the largest offsets reported for a long GRB. The bright optical afterglow, with dense coverage from COLIBRÍ, likely peaked at a few to several hours post-burst, followed by a shallow decay not expected from canonical afterglow models. Both the optical and X-ray light curves show a brief chromatic plateau from 4 − 7 days. We show that the subsequent rebrightening observed at ~20 days is best explained by the combined contribution of the associated Type Ic-BL supernova, identified in GTC spectra, and a late-time refreshed shock. The broadband optical to X-ray spectral energy distribution is well described by synchrotron emission from the forward shock, while the radio observations demand an additional emission component. We model the afterglow using (a) an on-axis uniform jet from a dirty fireball with late-time energy injection and (b) a misaligned jet with power-law angular structure, both having material emitting along our line-of-sight (LOS) moving with an initial Lorentz factor of Γ0 ~ 20 − 35. We conclude that at more typical GRB distances (z ≳ 0.5) the prompt γ-ray emission from this source would likely have escaped detection, whereas its optical afterglow would have remained observable, making the event appear as an orphan afterglow or a gamma-ray quiet fast X-ray transient.