Revisiting the propagation of highly-energetic gamma rays in the Galaxy

TL;DR

This paper investigates how the propagation of highly energetic gamma rays through the Galaxy affects observable signals, emphasizing the roles of background photon interactions and magnetic deflections, which are crucial for interpreting gamma-ray data and dark matter searches.

Contribution

The study provides detailed simulations of gamma-ray propagation in the Galaxy, highlighting how environmental factors influence observed spectra and directions, advancing understanding of gamma-ray astrophysics.

Findings

Propagation effects depend on source position and emission model.

Gamma-ray absorption features are identified in energy spectra.

Deflections alter the arrival directions of gamma rays.

Abstract

Recent gamma-ray observations have detected photons up to energies of a few PeV. These highly energetic gamma rays are emitted by the most powerful sources in the Galaxy. Propagating over astrophysical distances, gamma rays might interact with background photons producing electron-positron pairs, then deflected by astrophysical magnetic fields. In turn, these charged particles might scatter through inverse Compton galactic radiation fields, triggering electromagnetic cascades. In this scenario, the characterisation of astrophysical environment in which gamma rays travel, specifically background photons and magnetic fields, is crucial. We explore the impact of propagation effects on observables at Earth by simulating galactic sources emitting gamma rays with energy between $100 \; \text{GeV}$ and $100 \; \text{PeV}$. We analyse the imprint of the galactic environment on observed energy spectra and arrival direction maps, revealing gamma-ray absorption features in the former and ``deflection" of gamma rays in the latter. Specifically, owing to interstellar radiation field spatial distribution and the galactic magnetic field structure, propagation effects on observables are found to be related to the specific gamma-ray source position and to the prompt emission model. Detailed investigations of the propagation effect on galactic gamma rays will improve the robustness of both current and future gamma-ray detections and indirect dark matter searches.