Hadronic origin of prompt high-energy emission of gamma-ray bursts revisited: in the case of a limited maximum proton energy

TL;DR

This paper revisits the hadronic model for GRB high-energy emission, demonstrating that even with lower maximum proton energies, cascade processes can explain observed spectra and relax neutrino constraints.

Contribution

It shows that a lower maximum proton energy than previously assumed can still produce the observed high-energy emission in GRBs, emphasizing the importance of Bethe-Heitler processes.

Findings

Cascade emission can reproduce GRB high-energy spectra with lower proton energies.

Lower maximum proton energies relax constraints from non-detection of neutrinos.

Bethe-Heitler process can dominate over photomeson process at reduced proton energies.

Abstract

The high-energy (> 100MeV) emission observed by Fermi-LAT during the prompt phase of some luminous gamma-ray bursts (GRBs) could arise from the cascade induced by interactions between accelerated protons and the radiation field of GRBs. The photomeson process, which is usually suggested to operate in such a hadronic explanation, requires a rather high proton energy (> 10^17eV) for an efficient interaction. However, whether GRBs can accelerate protons to such a high energy is far from guaranteed, although they have been suggested as the candidate source for ultrahigh-energy cosmic rays. In this work, we revisit the hadronic model for the prompt high-energy emission of GRBs with a smaller maximum proton energy than the usually adopted value estimated from the Bohm condition. In this case, the Bethe-Heitler pair production process becomes comparably important or even dominates over the photomeson process. We show that with a relatively low maximum proton energy with a Lorentz factor of 10^5 in the comoving frame, the cascade emission can still reproduce various types of high-energy spectrum of GRBs. For most GRBs without high-energy emission detected, the maximum proton energy could be even lower and relax the constraints on the parameters of GRB jet resulting from the fact of non-detection of GRB neutrinos by IceCube.