GRB 260924A: Extreme Energetics, Bright GeV Emission, and Exceptional X-ray and Optical Luminosity
Abstract
We present a multiwavelength study of GRB~260924A at z=3.51±0.10 using \textit{Fermi}, \textit{Swift}, and ground-based optical and near-infrared observations. Its isotropic-equivalent gamma-ray energy, Eγ,iso≃9.79×1054~erg, places it among the most energetic GRBs observed. The prompt emission comprised two bright episodes separated by weaker activity. The X-ray afterglow ranked among the most luminous in our comparison sample at selected post-flare epochs, with a luminosity comparable to that of GRB~221009A at a rest-frame time of 3.5~ks. At a common redshift of z=1, the optical afterglow outshone GRB~221009A by approximately 2~mag at 0.3~d and 1~mag near 1~d. LAT emission extended to approximately 1100~s and showed an early excess above the subsequent decay. The highest-energy photon, 43.3~GeV, arrived at 82~s during the first episode; a 14.2~GeV photon arrived at 304~s between episodes. We describe the afterglow light curves through forward-shock synchrotron and synchrotron self-Compton (SSC) emission, with reverse-shock SSC accounting for the early LAT excess. We connect the early and late afterglow phases by modelling the X-ray rebrightening at several kiloseconds through energy injection from trailing ejecta and evolving shock microphysics. The late afterglow suggests a stratified medium, modelled as n(R)∝ R−1.2. Within this interpretation, the magnetic contrast RB≡(εB,r/εB,f)1/2 ≃2.6 suggests magnetized ejecta, while the highest-energy LAT photons exceed the predicted synchrotron limit, supporting an SSC contribution.
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BibTeX
@misc{fraija2026260924a,
title = {{GRB 260924A: Extreme Energetics, Bright GeV Emission, and Exceptional X-ray and Optical Luminosity}},
author = {Nissim Fraija and Peter Veres and Rosa L. Becerra and Antonio Galván-Gámez and Thomas Freeman and Simone Dichiara and Maria G Dainotti},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.32861},
eprint = {2610.32861},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.32861},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - GRB 260924A: Extreme Energetics, Bright GeV Emission, and Exceptional X-ray and Optical Luminosity
AU - Nissim Fraija
AU - Peter Veres
AU - Rosa L. Becerra
AU - Antonio Galván-Gámez
AU - Thomas Freeman
AU - Simone Dichiara
AU - Maria G Dainotti
PY - 2026
DA - 2026/10/23
PB - arXiv
DO - 10.48550/arxiv.2610.32861
UR - https://arxiv.org/abs/2610.32861
AB - We present a multiwavelength study of GRB~260924A at z=3.51±0.10 using \textit{Fermi}, \textit{Swift}, and ground-based optical and near-infrared observations. Its isotropic-equivalent gamma-ray energy, E_γ,iso≃9.79×10^54~erg, places it among the most energetic GRBs observed. The prompt emission comprised two bright episodes separated by weaker activity. The X-ray afterglow ranked among the most luminous in our comparison sample at selected post-flare epochs, with a luminosity comparable to that of GRB~221009A at a rest-frame time of 3.5~ks. At a common redshift of z=1, the optical afterglow outshone GRB~221009A by approximately 2~mag at 0.3~d and 1~mag near 1~d. LAT emission extended to approximately 1100~s and showed an early excess above the subsequent decay. The highest-energy photon, 43.3~GeV, arrived at 82~s during the first episode; a 14.2~GeV photon arrived at 304~s between episodes. We describe the afterglow light curves through forward-shock synchrotron and synchrotron self-Compton (SSC) emission, with reverse-shock SSC accounting for the early LAT excess. We connect the early and late afterglow phases by modelling the X-ray rebrightening at several kiloseconds through energy injection from trailing ejecta and evolving shock microphysics. The late afterglow suggests a stratified medium, modelled as n(R)∝ R^−1.2. Within this interpretation, the magnetic contrast R_B≡(ε_B,r/ε_B,f)^1/2 ≃2.6 suggests magnetized ejecta, while the highest-energy LAT photons exceed the predicted synchrotron limit, supporting an SSC contribution.
ER -