An extremely bright slow-rising afterglow from an off-axis jet in GRB 260310A
Abstract
We present a multi-wavelength study of GRB 260310A, a nearby long-duration gamma-ray burst at z≃0.153 associated with a broad-lined Type Ic supernova. Despite its modest prompt gamma-ray output, Eγ,iso≃3.5×1050 erg, GRB 260310A exhibits one of the brightest afterglows ever observed in the X-ray, optical, and radio bands. Its apparent brightness is not its only remarkable feature. The optical afterglow displays a delayed onset, characterized by a slow rising phase, with slope α≈−1, and a late peak at ≈0.1 d. We argue that the combination of weak prompt emission, hard peak energy, and late afterglow onset is naturally explained by a GRB jet viewed off-axis. The radio spectral energy distributions are consistent with synchrotron radiation and indicate the presence of both reverse- and forward-shock components, thus providing a first test of reverse-shock models in an off-axis geometry. The X-ray afterglow displays a prominent rebrightening, monitored for up to ≈68 d with no evidence of spectral evolution. A low level of linear polarization, Π≈1.7%, is measured at 15 GHz at T0+55 d and suggests that, at these late times, the forward-shock is the dominant emission component from radio to X-rays. This late-time rebrightening represents a critical test for the two-component jet model. If interpreted as the emergence of a narrow jet core viewed further off-axis, it would imply extreme luminosities and energetics for an on-axis observer.
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BibTeX
@misc{yang2026extremely,
title = {{An extremely bright slow-rising afterglow from an off-axis jet in GRB 260310A}},
author = {Yu-Han Yang and Roberto Ricci and Eleonora Troja and Muskan Yadav and Yi-Han Iris Yin and Rubén Sánchez-Ramírez and Brendan O'Connor and Niccoló Passaleva and Alberto J. Castro-Tirado and Hendrik van Eerten and Simone Dichiara and Vincenzo Galluzzi and Narjes Shahamat Dehsorkh and Iván Agudo and Jesús Aceituno and Malte Busmann and María D. Caballero-García and Emilio Fernández-García and Daniel Gruen and Maria Gritsevich and Sergiy Guziy and David Hiriart and You-Dong Hu and Martin Jelínek and Alexander Kutyrev and Alzbeta Malenakova and Filip Novotny and Ignacio Pérez-García and Shashi B. Pandey and Jorma Ryske and Alfredo Sota and Jan Strobl and Hira Waseem and Siyu Wu},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2605.23818},
eprint = {2605.23818},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.23818},
}
RIS
TY - UNPB
TI - An extremely bright slow-rising afterglow from an off-axis jet in GRB 260310A
AU - Yu-Han Yang
AU - Roberto Ricci
AU - Eleonora Troja
AU - Muskan Yadav
AU - Yi-Han Iris Yin
AU - Rubén Sánchez-Ramírez
AU - Brendan O'Connor
AU - Niccoló Passaleva
AU - Alberto J. Castro-Tirado
AU - Hendrik van Eerten
AU - Simone Dichiara
AU - Vincenzo Galluzzi
AU - Narjes Shahamat Dehsorkh
AU - Iván Agudo
AU - Jesús Aceituno
AU - Malte Busmann
AU - María D. Caballero-García
AU - Emilio Fernández-García
AU - Daniel Gruen
AU - Maria Gritsevich
AU - Sergiy Guziy
AU - David Hiriart
AU - You-Dong Hu
AU - Martin Jelínek
AU - Alexander Kutyrev
AU - Alzbeta Malenakova
AU - Filip Novotny
AU - Ignacio Pérez-García
AU - Shashi B. Pandey
AU - Jorma Ryske
AU - Alfredo Sota
AU - Jan Strobl
AU - Hira Waseem
AU - Siyu Wu
PY - 2026
DA - 2026/09/19
PB - arXiv
DO - 10.48550/arxiv.2605.23818
UR - https://arxiv.org/abs/2605.23818
AB - We present a multi-wavelength study of GRB 260310A, a nearby long-duration gamma-ray burst at z≃0.153 associated with a broad-lined Type Ic supernova. Despite its modest prompt gamma-ray output, E_γ,iso≃3.5×10^50 erg, GRB 260310A exhibits one of the brightest afterglows ever observed in the X-ray, optical, and radio bands. Its apparent brightness is not its only remarkable feature. The optical afterglow displays a delayed onset, characterized by a slow rising phase, with slope α≈−1, and a late peak at ≈0.1 d. We argue that the combination of weak prompt emission, hard peak energy, and late afterglow onset is naturally explained by a GRB jet viewed off-axis. The radio spectral energy distributions are consistent with synchrotron radiation and indicate the presence of both reverse- and forward-shock components, thus providing a first test of reverse-shock models in an off-axis geometry. The X-ray afterglow displays a prominent rebrightening, monitored for up to ≈68 d with no evidence of spectral evolution. A low level of linear polarization, Π≈1.7%, is measured at 15 GHz at T_0+55 d and suggests that, at these late times, the forward-shock is the dominant emission component from radio to X-rays. This late-time rebrightening represents a critical test for the two-component jet model. If interpreted as the emergence of a narrow jet core viewed further off-axis, it would imply extreme luminosities and energetics for an on-axis observer.
ER -