The effects of EBL inhomogeneity on the gamma-gamma absorption of VHE gamma-rays

Abstract

Very High Energy Gamma Rays (VHE; more than 100 GeV) from Cosmological Gamma Ray Sources such as blazars can be absorbed by the Extragalactic Background light (EBL), which leads to a high-energy cut-off in blazar spectral energy distributions. However, recent observations of distant gamma ray sources suggest that the universe is more transparent to VHE gamma rays than expected from our current knowledge of a homogeneous EBL. One of the possible solutions is the hypothesis that a reduced EBL opacity results from inhomogeneities of the EBL density in particular if the line of sight to a blazar is passing through large voids in intergalactic space. We have evaluated the inhomogeneous and anisotropic EBL density and resulting gamma-gamma opacity in such a case. We find that even a sizeable void $R \lesssim 1 \, h^{-1}$ ~Gpc or many typical voids $R \lesssim 100 \, h^{-1}$ ~Mpc located along the line of sight to a distant blazar leads to a reduction of the EBL opacity only around 15\%. EBL inhomogeneities are not expected to reduce the EBL gamma-gamma opacity significantly, and alternative solutions to the problem of hard VHE spectra of blazars may be required.