VHE gamma-rays from the hyper-bright GRB 221009A: implications for external inverse-Compton and proton synchrotron emission

The detection of the hyper-bright gamma-ray burst (GRB) 221009A provides an opportunity to investigate the nature of GRB emission and the origin of very-high-energy (VHE) gamma-rays. In this study, we examine the GeV-TeV emission within the framework of the external reverse-forward shock model. We observe that the external inverse-Compton mechanism, involving the upscattering of MeV gamma-rays by electrons accelerated at the external shock, is crucial in explaining the early emission in the range of approximately 1-200 GeV. This mechanism complements the synchrotron self-Compton component. Our results indicate the detectability of proton synchrotron emission from accelerated ultra-high-energy cosmic rays (UHECRs) and suggest that it could potentially account for the detection of photons at the O(10 TeV) level by LHAASO. Additionally, these findings may offer insights into constraining the mechanism responsible for UHECR acceleration.