Why the universe is unexpectedly transparent to very high energy gamma rays

Abstract

Recent observations of distant gamma ray sources indicate that the universe may be more transparent to VHE-gamma-rays than expected. In this paper we study the reduction of the EBL gamma-gamma opacity due to the existence of underdense regions along the line of sight to VHE gamma-ray sources and the possibility of a Lorentz Invariance Violation (LIV) signature. Therefore, we study whether one or both of these effects could be suitable to explain the spectral hardening observed in a few VHE gamma-ray sources. We found that, although the cosmic opacity for VHE gamma rays with energy more than 10 TeV can be strongly reduced, the spectral hardening feature observed in some VHE gamma-ray blazars with energy from 300 GeV up to few TeVs (e.g. PKS 1424+240) still remains puzzling.