Intergalactic $\gamma$-ray propagation: basic ideas, processes, and constraints

TL;DR

This paper reviews models of extragalactic gamma-ray propagation, focusing on electromagnetic cascades in the universe, and evaluates the potential of a new liquid Argon gamma-ray telescope for detecting such signals.

Contribution

It provides a detailed analysis of electromagnetic cascade development, angular distribution calculations, and assesses the sensitivity of a novel liquid Argon gamma-ray telescope.

Findings

Observable spectrum resembles universal EM cascade spectrum

Angular distribution calculations for nearby sources (<20 Mpc)

Liquid Argon telescope sensitivity exceeds Fermi LAT in 100 MeV–100 GeV range

Abstract

We review extragalactic $\gamma$-ray propagation models with emphasis on the electromagnetic (EM) cascade process in the magnetized expanding Universe. We consider cascades initiated by primary protons of ultra-high energy accelerated by blazars and show that the observable spectrum is similar to the universal spectrum of a purely EM cascade. We also present a detailed calculation of the observable angular distribution for the case of EM cascades developing from relatively nearby (<20 Mpc) sources. Finally, we calculate the point-like source differential sensitivity of a novel liquid Argon time projection chamber $\gamma$-ray telescope and show that its sensitivity is several times better than the Fermi LAT sensitivity in the 100 MeV -- 100 GeV energy range.