On conservative models of "the pair-production anomaly" in blazar spectra at Very High Energies

TL;DR

This paper reviews conservative models explaining the weaker-than-expected gamma-ray absorption features in some blazar spectra, focusing on intrinsic source spectra, cosmic-ray induced secondary photons, and electromagnetic cascades, without invoking new physics.

Contribution

It systematically evaluates existing conservative explanations for the pair-production anomaly in blazar spectra, highlighting the plausibility and limitations of each model.

Findings

Electromagnetic cascade model can reduce anomaly significance without conflicting constraints

Intrinsic source models fail to explain redshift dependence of the anomaly

Secondary photons from cosmic rays require highly collimated CR beams

Abstract

For some blazars, the gamma-ray absorption features due to pair-production on the Extragalactic Background Light (EBL) are fainter than expected. The present work reviews the main models that could explain this paradox, with emphasis on conservative ones, that do not include any new physics. The models that are intrinsic to the source, do allow a very hard primary spectrum, but fail to explain a regular redshift dependence of the anomaly starting energy. The model that includes a contribution from secondary photons produced by cosmic rays (CR) near the Earth seems to require a well collimated CR beam, what is hard to achieve. Finally, the model with secondary photons produced in electromagnetic (EM) cascades initiated by primary gamma-rays is considered. In principle, it allows to decrease the statistical significance of the anomaly and, while requiring quite low EGMF strength B, does not contradict to most contemporary constraints on the B value. Additionally, it is shown that the recently observed correlation between directions to hard gamma-ray sources and voids in the Large Scale Structure is a natural feature of the EM cascade model.