Deciphering the ~18 TeV photons from GRB 221009A

TL;DR

This paper investigates the detection of 18 TeV photons from GRB 221009A, proposing a model where high-energy protons interacting with synchrotron photons in the GRB's shock region explain the observed photons despite universe opacity.

Contribution

It introduces a novel scenario where high-energy proton interactions in the GRB afterglow account for ultra-high-energy photon observations.

Findings

High-energy photons up to 18 TeV can reach Earth despite universe opacity.

Proton interactions in the GRB shock region can produce detectable very high energy photons.

The model explains the unexpectedly high energy photon flux observed from GRB 221009A.

Abstract

On 9 October, 2022, an extremely powerful gamma-ray burst, GRB 221009A, was detected by several instruments. Despite being obstructed by the Milky Way galaxy, its afterglow outburst outshone all other GRBs seen before. LHAASO detected several thousands very high energy photons extending up to 18 TeV. Detection of such energetic photons are unexpected due to the large opacity of the Universe. It is possible that in the afterglow epoch the intrinsic very high energy photon flux from the source might have increased manifolds, which could compensate the attenuation by pair-production with the extragalactic background light. We propose such a scenario and show that very high energy photons can be observed on the Earth from the interaction of very high energy protons with the seed synchrotron photons in the external forward shock region of the GRB jet.