Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Upper Limits on the Extragalactic Background Light from the Gamma-Ray
  Spectra of Blazars   The direct measurement of the extragalactic background light (EBL) is
difficult at optical to infrared wavelengths because of the strong foreground
radiation originating in the Solar System. Very high energy (VHE, E$>$100 GeV)
gamma rays interact with EBL photons of these wavelengths through pair
production. In this work, the available VHE spectra from six blazars are used
to place upper limits on the EBL. These blazars have been detected over a range
of redshifts and a steepening of the spectral index is observed with increasing
source distance. This can be interpreted as absorption by the EBL. In general,
knowledge of the intrinsic source spectrum is necessary to determine the
density of the intervening EBL. Motivated by the observed spectral steepening
with redshift, upper limits on the EBL are derived by assuming that the
intrinsic spectra of the six blazars are $\propto E^{-1.8}$. Upper limits are
then placed on the EBL flux at discrete energies without assuming a specific
spectral shape for the EBL. This is an advantage over other methods since the
EBL spectrum is uncertain.

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The categories of this paper are: dg-ga
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The categories of this paper are: econ
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The categories of this paper are: hep-th
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The categories of this paper are: math-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: patt-sol
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The categories of this paper are: cond-mat
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The categories of this paper are: adap-org
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The categories of this paper are: ao-sci
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The categories of this paper are: q-alg
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The categories of this paper are: chao-dyn
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The categories of this paper are: cs
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The categories of this paper are: stat
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-ph
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The categories of this paper are: nlin