Secondary GeV-TeV emission from ultra-high-energy cosmic rays accelerated by GRB 221009A

Abstract

The origin of ultra-high-energy cosmic rays (UHECRs) remains elusive. Gamma-ray bursts (GRBs) are among the best candidates able to meet the stringent energy requirements needed for particle acceleration to such high energies. If UHECRs were accelerated by the central engine of GRB 221009A, it might be possible to detect secondary photons and neutrinos as the UHECRs travel from the source to the Earth. Here, we attempt to interpret some of the early publicly available data connected to this burst. If the reported early GeV-TeV detection was produced by secondary emission from UHECRs, it probably indicates that UHECRs reached energies > 10²¹ eV and that GRB 221009A exploded inside a magnetic void with intergalactic magnetic field (IGMF) strength B ≤ 3 × 10⁻¹⁶ G. In order to understand the entire energy deposition mechanism, we propose to search existing and future Fermi-LAT data for secondary emission arriving over larger spatial scales and longer time-scales. This strategy might help clarify the origin of UHECRs, constrain the IGMF strength along this line of sight, and start to quantify the fraction of magnetic voids around GRBs.