GRB 260310A/SN 2026fgk: a multiwavelength study of a nearby underluminous long GRB and SN with a complex afterglow
Abstract
We present a comprehensive multiwavelength study of GRB 260310A/SN 2026fgk, a nearby (z=0.153), long-duration gamma-ray burst (GRB) with an exceptionally underluminous prompt gamma-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 to 7 d. We show that the subsequent rebrightening observed at ∼20 d 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 broad-band 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 moving with an initial Lorentz factor of Γ0∼20−35. We conclude that at more typical GRB distances (z≳0.5) the prompt gamma-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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BibTeX
@article{gill2026260310asn,
title = {{GRB 260310A/SN 2026fgk: a multiwavelength study of a nearby underluminous long GRB and SN with a complex afterglow}},
author = {Ramandeep Gill and Rosa L Becerra and Antonio de Ugarte Postigo and Christina C Thöne and Alan M Watson and Noémie Globus and Jean-Grégoire Ducoin and Peter Veres and Stanley E Kurtz and Asuka Kuwata and Antonio Martín-Carrillo and Luca Izzo and Christophe Adami and Enrique Moreno Méndez and Nikos Mandarakas and Camila Angulo-Valdez and Stéphane Basa and William H Lee and Edilberto Aguilar-Ruiz and Dalya Akl and Margo F Aller and Miguel Ángel Aloy and Jie An and Sarah Antier and Jean-Luc Atteia and Nathaniel R Butler and Krittapas Chanchaiworawit and Philipe de la Parra and Damien Dornic and Francis Fortin and Shaoyu Fu and Johan P.U Fynbo and Lluis Galbany and Leonardo García-García and Stefan Geier and Marion Guelfand and Linbo He and Shuaiqing Jiang and Emeric Le Floc'H and Massimiliano Lincetto and Xing Liu and Gianluca Lombardi and Diego López-Cámara and Daniele Bjørn Malesani and Francesco Magnani and Kanthanakorn Noysena and Margarita Pereyra and Ny Avo Rakotondrainibe and Anthony C.S Readhead and Delphine Russeil and Fredd Sánchez-Álvarez and Benjamin Schneider and Tirth D Surti and Nial R Tanvir and Samaporn Tinyanont and Dong Xu and Zipei Zhu},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2026},
month = aug,
doi = {10.1093/mnras/stag1544},
eprint = {2605.26091},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05743313v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - GRB 260310A/SN 2026fgk: a multiwavelength study of a nearby underluminous long GRB and SN with a complex afterglow
AU - Ramandeep Gill
AU - Rosa L Becerra
AU - Antonio de Ugarte Postigo
AU - Christina C Thöne
AU - Alan M Watson
AU - Noémie Globus
AU - Jean-Grégoire Ducoin
AU - Peter Veres
AU - Stanley E Kurtz
AU - Asuka Kuwata
AU - Antonio Martín-Carrillo
AU - Luca Izzo
AU - Christophe Adami
AU - Enrique Moreno Méndez
AU - Nikos Mandarakas
AU - Camila Angulo-Valdez
AU - Stéphane Basa
AU - William H Lee
AU - Edilberto Aguilar-Ruiz
AU - Dalya Akl
AU - Margo F Aller
AU - Miguel Ángel Aloy
AU - Jie An
AU - Sarah Antier
AU - Jean-Luc Atteia
AU - Nathaniel R Butler
AU - Krittapas Chanchaiworawit
AU - Philipe de la Parra
AU - Damien Dornic
AU - Francis Fortin
AU - Shaoyu Fu
AU - Johan P.U Fynbo
AU - Lluis Galbany
AU - Leonardo García-García
AU - Stefan Geier
AU - Marion Guelfand
AU - Linbo He
AU - Shuaiqing Jiang
AU - Emeric Le Floc'H
AU - Massimiliano Lincetto
AU - Xing Liu
AU - Gianluca Lombardi
AU - Diego López-Cámara
AU - Daniele Bjørn Malesani
AU - Francesco Magnani
AU - Kanthanakorn Noysena
AU - Margarita Pereyra
AU - Ny Avo Rakotondrainibe
AU - Anthony C.S Readhead
AU - Delphine Russeil
AU - Fredd Sánchez-Álvarez
AU - Benjamin Schneider
AU - Tirth D Surti
AU - Nial R Tanvir
AU - Samaporn Tinyanont
AU - Dong Xu
AU - Zipei Zhu
PY - 2026
DA - 2026/08/18
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/stag1544
UR - https://hal.science/hal-05743313v1
AB - We present a comprehensive multiwavelength study of GRB 260310A/SN 2026fgk, a nearby (z=0.153), long-duration gamma-ray burst (GRB) with an exceptionally underluminous prompt gamma-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 to 7 d. We show that the subsequent rebrightening observed at ∼20 d 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 broad-band 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 moving with an initial Lorentz factor of Γ0∼20−35. We conclude that at more typical GRB distances (z≳0.5) the prompt gamma-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.
SN - 0035-8711
SN - 1365-2966
ER -