Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A low density of the Extragalactic Background Light revealed by the
  H.E.S.S. spectra of the BLLac objects 1ES 1101-232 and H 2356-309   The unexpectedly hard spectra measured by HESS for the BLLacs 1ES 1101-232
and H 2356-309 has allowed an upper limit on the Extragalactic Background Light
(EBL) to be derived in the optical/near-infrared range, which is very close to
the lower limit given by the resolved galaxy counts. This result seems to
exclude a large contribution to the EBL from other sources (e.g. Population III
stars) and indicates that the intergalactic space is more transparent to
gamma-rays than previously thought. A brief discussion of EBL absorption
effects on blazar spectra and further observational tests to check this
conclusion are presented, including the selection of new candidates for
observations with Cherenkov telescopes.

2
The categories of this paper are: math
1
The categories of this paper are: cs
27
The categories of this paper are: comp-gas
24
The categories of this paper are: dg-ga
22
The categories of this paper are: chao-dyn
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
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The categories of this paper are: physics
19
The categories of this paper are: eess
8
The categories of this paper are: hep-ex
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
33
The categories of this paper are: plasm-ph
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph