Multi-TeV $\gamma$-ray candidates from GRB 221009A: a downturn in the intrinsic $\gamma$-ray spectrum, an echo of the prompt emission phase, and intergalactic electromagnetic cascades

TL;DR

This paper analyzes multi-TeV gamma-ray data from GRB 221009A, revealing a spectral downturn, proposing neutron-induced echoes of prompt emission, and assessing the limited role of intergalactic cascades at these energies.

Contribution

It provides the first detailed reconstruction of the intrinsic gamma-ray spectrum of GRB 221009A, identifies a spectral downturn, and models high-energy echoes and cascade contributions.

Findings

Spectral downturn at several TeV with >5 sigma significance.

Neutron interactions produce observable high-energy gamma-ray echoes.

Intergalactic electromagnetic cascades are limited during early afterglow for B > 1 nG.

Abstract

The detection of $\gamma$-ray candidates up to the energy of $\approx$13 TeV from the exceptionally bright $\gamma$-ray burst GRB 221009A by the Large High Altitude Air-Shower Observatory (LHAASO) has raised considerable interest in the astrophysical community. The $\gamma$-ray dataset resulting from the LHAASO observations allows one to reconstruct the intrinsic spectrum of GRB 221009A with an unprecedented precision. This intrinsic spectrum reveals a downturn at the energy of several TeV (statistical significance $\gt 5 \sigma$), i.e. the reconstructed intensity is below the intensity expected for a power-law spectrum. We show that a significant TeV $\gamma$-ray component may be produced by neutrons from photohadronic interactions inside the fireball. These neutrons escape the fireball and interact with the surrounding matter, giving rise to a flux of electrons and positrons, eventually resulting in an observable flux of GeV--TeV synchrotron photons -- a high energy "echo" of the GRB prompt emission phase. Finally, we show that at multi-TeV energies the contribution of $\gamma$ rays from intergalactic electromagnetic cascades initiated by primary ultra high energy protons is severely limited during the early afterglow phase for the typical magnetic field strength in the intergalactic filaments above 1 nG.