Photon propagation and the VHE gamma-ray spectra of blazars: how transparent is really the Universe?

TL;DR

This paper explores how photon-axion-like particle mixing could explain the unexpectedly high transparency of the universe to very-high-energy gamma rays from distant blazars, aligning observations with theoretical models.

Contribution

It introduces the DARMA scenario as a natural explanation for gamma-ray transparency and predicts redshift independence of blazar spectral indices at high energies.

Findings

DARMA explains the observed gamma-ray transparency.

Spectral slope becomes redshift-independent for distant sources.

Predictions are testable with current gamma-ray observatories.

Abstract

Recent findings by gamma-ray Cherenkov telescopes suggest a higher transparency of the Universe to very-high-energy (VHE) photons than expected from current models of the Extragalactic Background Light. It has been shown that such transparency can be naturally explained by the DARMA scenario, in which the photon mixes with a new, very light, axion-like particle predicted by many extensions of the Standard Model of elementary particles. We discuss the implications of DARMA for the VHE gamma-ray spectra of blazars, and show that it successfully accounts for the observed correlation between spectral slope and redshift by adopting for far-away sources the same emission spectrum characteristic of nearby ones. DARMA also predicts the observed blazar spectral index to become asymptotically independent of redshift for far-away sources. Our prediction can be tested with the satellite-borne Fermi/LAT detector as well as with the ground-based Cherenkov telescopes HESS, MAGIC, CANGAROOIII, VERITAS and the Extensive Air Shower arrays ARGO-YBJ and MILAGRO.