External Inverse-compton and Proton Synchrotron Emission from the Reverse Shock as the Origin of VHE Gamma Rays from the Hyper-bright GRB 221009A

The detection of the hyper-bright gamma-ray burst (GRB) 221009A enables us to explore the nature of the GRB emission and the origin of very high-energy gamma rays. We analyze the Fermi Large Area Telescope (Fermi-LAT) data of this burst and investigate the GeV-TeV emission in the framework of the external reverse-shock model. We show that the early similar to 1-10 GeV emission can be explained by the external inverse-Compton mechanism via upscattering MeV gamma rays by electrons accelerated at the reverse shock, in addition to the synchrotron self-Compton component. The predicted early optical flux could have been brighter than that of the naked-eye GRB 080319B. We also show that proton synchrotron emission from accelerated ultrahigh-energy cosmic rays (UHECRs) is detectable and could potentially explain greater than or similar to TeV photons detected by LHAASO or constrain the UHECR acceleration mechanism. Our model suggests that the detection of 0(10 TeV) photons with energies up to similar to 18 TeV is possible for reasonable models of the extragalactic background light without invoking new physics and predicts anticorrelations between MeV photons and TeV photons, which can be tested with the LHAASO data.

Automated summary

The detection of hyper-bright gamma-ray burst (GRB) 221009A allows us to investigate the nature of GRB emission and the origin of very high-energy gamma rays. Using Fermi Large Area Telescope (Fermi-LAT) data, we analyze the GeV-TeV emission of this burst within the framework of the external reverse-shock model. Our findings suggest that the early emission in the 1-10 GeV range can be explained by the external inverse-Compton mechanism, in addition to the synchrotron self-Compton component. We also propose that the proton synchrotron emission from accelerated ultrahigh-energy cosmic rays (UHECRs) is detectable and may explain TeV photons detected by LHAASO or constrain the UHECR acceleration mechanism.