Abstract
Most of the recent research on extragalactic γ-ray propagation focused on the study of the γγ * e+e− absorption process (“absorption-only model”). Starting from a possible anomaly at very high energies (VHE, E > 100 GeV), we briefly review several existing deviations from this model. The exotic interpretation of the VHE anomaly is not supported by the recent works. On the other hand, the process of intergalactic electromagnetic cascade development naturally explains these effects. We discuss phenomenology of intergalactic cascades and the main spectral signatures of the electromagnetic cascade model. We also briefly consider the hadronic cascade model; it also may explain the data, but requires low strength of magnetic field around the source of primary protons or nuclei.
Highlights
Observations made with imaging Cherenkov telescopes in the VHE range provide an opportunity to test extragalactic gamma-ray propagation models (e.g. [1, 2])
It is evident that without the artificial cutoff of the extragalactic background light (EBL) spectral density at low energies the suppression in Re(E B L) is absent. While this spectral feature is clearly irrelevant for the final result on the observable spectrum of cascade γ -rays, it may appear of use while testing precision numerical Monte Carlo (MC) codes aimed at intergalactic cascade simulations
The first statistically significant (> 3σ ) deviation from the absorption-only model was the VHE anomaly, that was initially interpreted as an indication for the γ → AL P oscillations
Summary
Observations made with imaging Cherenkov telescopes in the VHE range provide an opportunity to test extragalactic gamma-ray propagation models (e.g. [1, 2]). The existence of the anomaly was initially interpreted as evidence for γ –axion-like particle (γ → AL P) oscillations [2]. This model is briefly considered in Sect. There are some other and less exotic ways to account for this anomaly These explanations include questioning the validity of the theoretical EBL models invoked to explain observations. While the change of the EBL model could decrease the statistical significance of the anomaly at high energies, it certainly does not explain the whole observation set The anomaly can be explained in the framework of the electromagnetic (Sect. 4) or hadronic (Sect. 5) intergalactic cascade models
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