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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 catalogue 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 Fermi Large Area Telescope hard source catalogue (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 Fermi hard source population (1.7σ). A correlation between 10 and 500 GeV flux and underdense fraction along the line of sight for VHE-like and Fermi 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 Fermi catalogue, containing sources detected above 100 MeV, or the SDSS Data Release 7 quasar catalogue. 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 per cent 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 attenuation of gamma-rays decreases no more than 10 per cent. This decrease, although non-negligible, is unable to account for the apparent hard source correlation with underdense lines of sight.
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Submitted on : Thursday, April 28, 2022 - 5:02:22 PM
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A. Furniss, P. M. Sutter, J. R. Primack, A. Domínguez. A correlation between hard gamma-ray sources and cosmic voids along the line of sight. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2015, 446, pp.2267-2273. ⟨10.1093/mnras/stu2196⟩. ⟨insu-03644785⟩

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