Deciphering the ∼18 TeV Photons from GRB 221009A

On 2022 October 9, 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 thousand very high energy photons extending up to 18 TeV. Detection of such energetic photons is 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 for 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.