GRB 260310A / SN 2026fgk: A Multi-Wavelength Study of a Nearby Underluminous Long GRB and SN with a Complex Afterglow

Abstract 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.

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Journal
Monthly Notices of the Royal Astronomical Society
Published
2026-08-14
DOI
https://doi.org/10.1093/mnras/stag1544
Primary Topic
Gamma-ray bursts and supernovae
Type
article
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article

GRB 260310A / SN 2026fgk: A Multi-Wavelength Study of a Nearby Underluminous Long GRB and SN with a Complex Afterglow

Diego López-Cámara, K. Noysena, N. Butler, Noémie Globus et al.
Monthly Notices of the Royal Astronomical Society
Gamma-ray bursts and supernovae
article

GRB 260310A / SN 2026fgk: A Multi-Wavelength Study of a Nearby Underluminous Long GRB and SN with a Complex Afterglow

Diego López-Cámara, K. Noysena, N. Butler, Noémie Globus, D. Russeil, Dalya Akl, Edilberto Aguilar-Ruiz, J. L. Atteia, Asuka Kuwata, Enrique Moreno Méndez, Krittapas Chanchaiworawit, Alan M. Watson, Stefan Geier, Leonardo García-García, S. Kurtz, C. C. Thöne, Margarita Pereyra, P. V. de la Parra, Camila Angulo-Valdez, N.A Rakotondrainibe, C. Adami, Fredd Sánchez-Álvarez, Nikos Mandarakas, Shuaiqing Jiang, Xing Liu, Linbo He, Luca Izzo, Jean-Grégoire Ducoin, Stéphane Basa, Damien Dornic, Rosa L. Becerra, Marion Guelfand, Francesco Magnani, Lluis Galbany, Zipei Zhu, William H. Lee, Sarah Antier, Peter Veres, Miguel Ángel Aloy, Jie An, Antonio Martín-Carrillo, Anthony C. S. Readhead, Tirth D. Surti, Francis Fortin, Dong Xu, Emeric Le Floc'h, Benjamin Schneider, Samaporn Tinyanont, Massimiliano Lincetto, Margo F. Aller, Daniele Bjørn Malesani, Ramandeep Gill, Nial R. Tanvir, Shaoyu Fu, Johan P. U. Fynbo, Antonio de Ugarte Postigo, Gianluca Lombardi
article en

Abstract

Abstract 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.

Monthly Notices of the Royal Astronomical Society
University College Dublin (IE), New York University Abu Dhabi (AE), California Institute of Technology (US), University of Copenhagen (DK), Centre National de la Recherche Scientifique (FR), Institute of Space Sciences (ES), Radboud University Nijmegen (NL), University of Leicester (GB), Universitat de València (ES), Open University of Israel (IL), Universidad Autónoma de la Ciudad de México (MX), University of Concepción (CL), Universidad Autónoma de Baja California (MX), Université Fédérale de Toulouse Midi-Pyrénées (FR), Laboratoire d'Analyse et d'Architecture des Systèmes (FR), Chinese Academy of Sciences (CN), Université Paris Cité (FR), Aix-Marseille Université (FR), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris-Saclay (FR), University of Michigan (US), Parc Científic de la Universitat de València (ES), Astronomical Observatory of Capodimonte (IT), Evgeni Kharadze Georgian National Astrophysical Observatory (GE), Astrophysique, Instrumentation et Modélisation (FR), Institut d'Estudis Espacials de Catalunya (ES), DAWN Center for Independent Living (US), Altaiskiy State Nature Biosphere Reserve (RU), Secretaría de Ciencia Tecnología e Innovación (MX), CEA Paris-Saclay (FR), Instituto de Astrofísica de Canarias (ES), Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (FR), Gran Telescopio Canarias (Spain) (ES), National Astronomical Observatories (CN), Huazhong University of Science and Technology (CN), Arizona State University (US), University of Alabama in Huntsville (US), National Astronomical Research Institute of Thailand (TH), Foundation for Research and Technology Hellas (GR), Universidad Nacional Autónoma de México (MX)
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Gamma-ray bursts and supernovae
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