EBL Inhomogeneity and Hard-Spectrum Gamma-Ray Sources

Abstract

The unexpectedly hard very-high-energy (VHE; $E>100$ GeV) $\gamma$-ray spectra of a few distant blazars have been interpreted as evidence for a reduction of the $\gamma \gamma$ opacity of the Universe due to the interaction of VHE $\gamma$-rays with the extragalactic background light (EBL) compared to the expectation from our current knowledge of the density and cosmological evolution of the EBL. One of the suggested solutions to this problem consisted of the inhomogeneity of the EBL. In this paper, we study the effects of such inhomogeneities on the energy density of the EBL (which then also becomes anisotropic) and the resulting $\gamma \gamma$ opacity. Specifically, we investigate the effects of cosmic voids along the line of sight to a distant blazar. We find that the effect of such voids on the $\gamma \gamma$ opacity, for any realistic void size, is only of the order of $\lesssim 1$ % and much smaller than expected from a simple linear scaling of the $\gamma \gamma$ opacity with the line-of-sight galaxy under-density due to a cosmic void.

arXiv commentAccepted for Publication in The Astrophysical Journal

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• The Gamma-Ray Window to Intergalactic Magnetism
• White Paper and Roadmap for Quantum Gravity Phenomenology in the Multi-Messenger Era
• Lorentz Invariance Violation Effects on Gamma-Gamma Absorption and Compton Scattering
• Testing CIBER cosmic infrared background measurements and axionlike particles with observations of TeV blazars
• Energetic particles and high-energy processes in cosmological filaments and their astronomical implications
• Constraints on Extragalactic Background Light using TeV Observations of BL Lacertae objects
• Detailed modelling of the EBL along VHE γ-ray paths
• Impacts of Voids, Line of Sight Interactions, and Local Emission Environment on Detectability of Gamma-Ray AGN