Detailed modelling of the EBL along VHE {\gamma}-ray paths

TL;DR

This paper develops a hybrid model of the extragalactic background light (EBL) to study its impact on very high energy gamma-ray spectra, revealing that local EBL fluctuations have minimal effect, but massive galaxies near the line of sight can significantly influence gamma-ray absorption.

Contribution

The study introduces a new hybrid EBL model combining analytic and galaxy catalog data, enabling detailed analysis of gamma-ray opacity and local EBL fluctuations along individual paths.

Findings

Local EBL fluctuations are about an order of magnitude but have negligible impact on gamma-ray opacity.

Galaxies with stellar mass >10^{11} M_sun can significantly affect gamma-ray absorption.

Proximity of single massive galaxies is unlikely to explain observed spectral index variations.

Abstract

Interactions between the extragalactic background light (EBL) and very high energy {\gamma} rays (VHE; E > 10 GeV) from cosmological sources alter their {\gamma}-ray spectrum. The stronger absorption of harder {\gamma} rays causes a steepening of the observed {\gamma}-ray spectrum at the high energy tail which can be expressed by an increase of the power index. The effect provides a link between high energy astrophysics and the evolution of galaxies. In this work we develop a new hybrid EBL model by augmenting our previous analytic model with information from semi-analytic galaxy catalogues. The model allows us to study the {\gamma}-ray opacities of individual {\gamma}-ray paths through the (simulated) universe and evaluate the effect of local fluctuations in the EBL intensity along each path. We confirm an order of magnitude fluctuations in the local EBL (based on the cumulative light from model galaxy distributions within spheres of 50 $h^{-1}$Mpc). However, the effect of these fluctuations on the VHE {\gamma}-ray opacity is insignificant due to the overall very small contribution of the local EBL to the total EBL. We also investigate the effect which a galaxy crossing the line of sight of the {\gamma}-ray source may have on the VHE spectrum. We find that galaxies with stellar masses of M$ > 10^{11}$ M$_{\odot}$ could have significant effect on the {\gamma}-ray absorption. It is unlikely that the observed variation in the spectral index is caused by the proximity of single galaxies to the {\gamma}-ray paths as these are extremely rare.