GRB 221009A: a potential source of ultra-high-energy cosmic rays

Abstract

Recently an extraordinarily bright gamma-ray burst, GRB 221009A, was observed by several facilities covering the whole electromagnetic spectrum. Gamma rays with energies up to 18 TeV were detected, as well as a possible photon with 251 TeV. Such energetic events are not expected because they would be attenuated by pair-production interactions with the extragalactic background light. This tension is, however, only apparent, and does not call for any unconventional explanation. Here I show that these observations can be interpreted as the result of ultra-high-energy cosmic rays (UHECRs) interacting with cosmological radiation fields during their journey to Earth, provided that intergalactic magnetic fields are reasonably weak. If this hypothesis is correct, it would establish bursts like GRB 221009A as UHECR sources.