# Testing models of extragalactic $\gamma$-ray propagation using observations of extreme blazars in GeV and TeV energy ranges

## TL;DR

This paper reviews models of extragalactic gamma-ray propagation, highlighting the uncertain magnetic fields and supporting the electromagnetic cascade model through observational data from Cherenkov telescopes and Fermi LAT.

## Contribution

It provides a synthesis of current models and observational evidence for electromagnetic cascades in extragalactic gamma-ray propagation.

## Key findings

- Lower limits on EGMF strength in voids are established.
- Electromagnetic cascade model is supported by observational data.
- Secondary gamma-ray component contributes significantly to observed spectra.

## Abstract

We briefly review contemporary extragalactic {\gamma}-ray propagation models. It is shown that the Extragalactic Magnetic Field (EGMF) strength and structure are poorly known. Strict lower limits on the EGMF strength in voids are of order 10^{-17}--10^{-20} G, thus allowing a substantial contribution of a secondary component generated by electromagnetic cascades to the observable spectrum. We show that this "electromagnetic cascade model" is supported by data from imaging Cherenkov telescopes and the Fermi LAT detector.