Local absorption of high-energy emission from gamma-ray bursts

TL;DR

This paper evaluates the absorption of high-energy gamma-ray photons from gamma-ray bursts by various photon fields, concluding that galactic and extragalactic backgrounds are the primary sources of opacity at energies above 10 TeV.

Contribution

The study provides detailed estimates of photon-photon opacity from local and galactic photon fields, emphasizing the dominance of extragalactic background light at very high energies.

Findings

Photon fields near GRBs are mostly transparent for GeV energies.

Galactic radiation fields cause minimal opacity below 10 TeV.

Extragalactic background light dominates opacity at energies above 10 TeV.

Abstract

High-energy photons emitted from gamma-ray bursts (GRBs) are subject to pair-production interactions with lower energy photons, leading to an effective optical depth. In this Letter, we estimate the opacity resulting from photon fields located at various distances from long GRB sites: that of the binary companion to the massive stellar progenitor, that of the star-forming molecular cloud containing the GRB, and the total photon field of the host galaxy. The first two photon fields are found to be transparent for most reasonable sets of assumptions about these systems. In the case of galactic radiation fields, we have performed several numerical simulations to calculate the expected opacities for different line-of-sight geometries through the host galaxy, and include a full accounting of the infrared radiation produced by the absorption and re-radiation of starlight by dust. The optical depth for GeV gamma-rays, due to direct starlight is less than unity for all host galaxies. At higher energies, $>$10 TeV, a spectral cutoff can occur due to the rapidly increasing number of mid- to far-IR intra-galactic photons reradiated by dust. Photons in the extragalactic background light therefore remain the only relevant source of photon-photon opacity for ongoing GRB observations with Fermi LAT, and potential future detections with ground-based gamma-ray telescopes.