Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A Correlation Between Hard Gamma-ray Sources and Cosmic Voids Along the
  Line of Sight   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 Fermi Large Area Telescope hard
source catalog (together referred to as "VHE-like" sources) are distributed
along underdense lines of sight at the 2.4 sigma level. There is also a less
suggestive correlation for the Fermi hard source population (1.7 sigma). A
correlation between 10-500 GeV flux and underdense fraction along the line of
sight for VHE-like and Fermi hard sources is found at 2.4 sigma and 2.6 sigma,
respectively. The preference for underdense sight lines is not displayed by
gamma-ray emitting galaxies within the second Fermi 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 tau(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.

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The categories of this paper are: hep-ph
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
4
The categories of this paper are: cond-mat
9
The categories of this paper are: nlin
27
The categories of this paper are: comp-gas
11
The categories of this paper are: hep-lat
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
1
The categories of this paper are: cs
22
The categories of this paper are: chao-dyn
20
The categories of this paper are: acc-phys
5
The categories of this paper are: gr-qc
12
The categories of this paper are: nucl-th
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The categories of this paper are: dg-ga
21
The categories of this paper are: adap-org