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 observa- tions 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 so- lutions 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 . 1 h − 1 Gpc or many typical voids R . 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