Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: An Upper Limit on the Infrared Background Density from HEGRA data on
  Mkn501   The energy spectrum of Mkn501 in the TeV energy regime, as measured by the
HEGRA (High Energy Gamma Ray Astronomy) Cerenkov telescopes during its low
state in 1995/96 and during a fraction of the 1997 outburst in the TeV energy
regime, is shown to place stringent upper limits on the still unknown infrared
photon density in the energy region between 3 x 10**(-3) and 3 xt 10**(-1) eV.
Assuming two different shapes for the unknown infrared photon spectrum in this
energy range we calculate upper limits on the infrared photon density on the
basis of the power-law fit obtained for the observed spectrum up to the maximum
energy.

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The categories of this paper are: adap-org
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The categories of this paper are: math-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
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The categories of this paper are: solv-int
35
The categories of this paper are: mtrl-th
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The categories of this paper are: q-bio
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The categories of this paper are: hep-lat
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The categories of this paper are: physics
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The categories of this paper are: nucl-ex
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The categories of this paper are: nucl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: math