A correlation between hard gamma-ray sources and cosmic voids along the line of sight

Abstract

We estimate the galaxy density along lines of sight to hard extragalactic gamma-ray sources by correlating source positions on the sky with a void catalog based on the Sloan Digital Sky Survey (SDSS). Extragalactic gamma-ray sources that are detected at very high energy (VHE; E > 100 GeV) or have been highlighted as VHE-emitting candidates in the F ermi Large Area Telescope hard source catalog (together referred to as "VHE-like" sources) are distributed along underdense lines of sight at the 2.4σ level. There is also a less suggestive correlation for the F ermi hard source population (1.7σ). A correlation between 10-500 GeV flux and underdense fraction along the line of sight for VHE-like and F ermi hard sources is found at 2.4σ and 2.6σ, calculated from the Pearson correlation coefficients of r = 0.57 and 0.47, respectively. The preference for underdense sight lines is not displayed by gamma-ray emitting galaxies within the second F ermi catalog, containing sources detected above 100 MeV, or the SDSS DR7 quasar catalog. We investigate whether this marginal correlation might be a result of lower extragalactic background light (EBL) photon density within the underdense regions and find that, even in the most extreme case of a entirely underdense sight line, the EBL photon density is only 2% less than the nominal EBL density. Translating this into gamma-ray attenuation along the line of sight for a highly attenuated source with opacity τ (E, z) ∼ 5, we estimate that the attentuation of gamma-rays decreases no more than 10%. This decrease, although non-neglible, is unable to account for the apparent hard source correlation with underdense lines of sight.

Measurement of the EBL spectral energy distribution using the VHE γ-ray spectra of H.E.S.S. blazars

“…Individual EBL levels should essentially be compatible with each other as the EBL is assumed to be a diffuse isotropic background. Potential anisotropies of the EBL (Furniss et al 2015;Abdalla & Böttcher 2017) are estimated to be beyond the sensitivity of this EBL measurement method. The dispersion of individual values can reflect systematic uncertainties and potential limitations of the procedure.…”

EBL Inhomogeneity and Hard-Spectrum Gamma-Ray Sources

“…Possible solutions include the hypothesis that the EBL density is generally lower than expected from current models (Furniss et al 2013); the existence of exotic axion like particles (ALPs) into which VHE γ-rays can oscillate in the presence of a magnetic field, thus enabling VHE-photons to avoid γγ absorption (Dominguez et al 2011b); an additional VHE γ-ray emission component due to interactions along the line of sight of extragalactic ultra-high-energy cosmic rays (UHECRs) originating from the blazar (e.g., Essey & Kusenko 2010); and EBL inhomogeneities. The idea of EBL inhomogeneities was considered by Furniss et al (2015), who found tentative hints for correlations between hard VHE γ-ray sources and under-dense regions along the line of sight.…”

The VHE anomaly in blazar spectra and related phenomena

“…For instance, the authors of [26] proposed some alternative explanations to their findings in the paper itself. The significance of the effect observed in [28] is below 3 σ. As for [29], the result could in principle be explained by the scattering of electrons and positrons accelerated in the source by a stronger local magnetic field (B ∼ 10 −12 − 10 −7 G), in which case quasi-isotropic halos may form around the source [30].…”

“…Finally, fig. 13 presents several curves of the boost factor vs. energy labeled by different values of the fraction of space filled with voids -the so-called "voidiness" parameter [28]. It is interesting that at comparatively low values of voidiness (0.2-0.3) the values of K B are much larger at high energies than for the case of voidiness =1.…”