Ultrahigh-energy cosmic-ray signature in GRB 221009A

TL;DR

This paper investigates the high-energy gamma-ray emission from GRB 221009A, suggesting that ultrahigh-energy cosmic rays accelerated in the burst's blastwave can explain the observed >10 TeV gamma rays, challenging conventional leptonic models.

Contribution

It proposes a novel explanation involving ultrahigh-energy cosmic rays in GRBs to account for >10 TeV gamma-ray observations, beyond standard leptonic models.

Findings

Leptonic models cannot fully explain >10 TeV gamma rays from GRB 221009A.

Ultrahigh-energy cosmic rays in the GRB blastwave can produce electromagnetic cascades.

The observed >10 TeV emission may indicate cosmic-ray acceleration in GRBs.

Abstract

The brightest long gamma-ray burst detected so far by the Swift-BAT and Fermi-GBM telescopes, GRB~221009A, provides an unprecedented opportunity for understanding the high-energy processes in extreme transient phenomena. We find that the conventional leptonic models, synchrotron and synchrotron-self-Compton, for the afterglow emission from this source have difficulties explaining the observation of $\gtrsim 10$ TeV $\gamma$ rays, by the LHAASO detector, and extending up to 18 TeV energies. We model $\gamma$-ray spectrum estimated in the energy range 0.1-1 GeV by the Fermi-LAT detector. The flux predicted by our leptonic models is severely attenuated at $> 1$ TeV due to $\gamma\gamma$ pair production with extragalactic background light, and hence an additional component is required at $\gtrsim 10$ TeV. Ultrahigh-energy cosmic rays can be accelerated in the GRB blastwave, and their propagation induces an electromagnetic cascade in the extragalactic medium. The line of sight component of this flux can explain the emission at $\gtrsim 10$ TeV detected by LHAASO, requiring a fraction of the GRB blastwave energy in ultrahigh-energy cosmic rays. This could be an indication of ultrahigh-energy cosmic-ray acceleration in GRBs.