The origin of very-high-energy gamma-rays from GRB 221009A: implications for reverse shock proton synchrotron emission

TL;DR

This paper proposes a two-component jet model to explain the very-high-energy gamma-ray emission from GRB 221009A, suggesting reverse shock proton synchrotron emission as a key mechanism and its potential role in accelerating ultra-high-energy cosmic rays.

Contribution

It introduces a structured jet model combining forward shock and reverse shock emissions to explain VHE gamma-ray observations, highlighting reverse shock proton synchrotron as a novel explanation.

Findings

Reverse shock proton synchrotron emission explains VHE flux and spectral evolution.

Arrival time of highest energy photons coincides with reverse shock emission peak.

GRB reverse shock may accelerate ultra-high-energy cosmic rays.

Abstract

Recently, GRB 221009A, known as the brightest of all time (BOAT), has been observed across an astounding range of $\sim 18$ orders of magnitude in energy, spanning from radio to VHE bands. Notably, the Large High Altitude Air Shower Observatory (LHAASO) recorded over $60000$ photons with energies exceeding $0.2\rm~TeV$, including the first-ever detection of photons above $10\rm~TeV$. However, explaining the observed energy flux evolution in the VHE band alongside late-time multi-wavelength data poses a significant challenge. Our approach involves a two-component structured jet model, consisting of a narrow core dominated by magnetic energy and a wide jet component dominated by matter. We show that the combination of the forward shock electron synchrotron self-Compton emission from both jets and reverse shock proton synchrotron emission from the wide jet could account for both the energy flux and spectral evolution in the VHE band, and the early TeV lightcurve may be influenced by prompt photons which could explain the initial steep rising phase. We noticed the arrival time of the highest energy photons detected by LHAASO-KM2A coincident with the peak of the reverse shock proton synchrotron emission, especially a minor flare occurring about $\sim500-800$ seconds after the trigger, coinciding with the observed spectral hardening and arrival time of the $\sim 13\rm~TeV$ photons detected by LHAASO. These findings imply that the GRB reverse shock may serve as a potential accelerator of ultra-high-energy cosmic rays, a hypothesis that could be tested through future multimessenger observations.