Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: TeV Observations of the Variability and Spectrum of Markarian 501   Markarian 501 is only the second extragalactic source to be detected with
high statistical certainty at TeV energies; it is similar in many ways to
Markarian 421. The Whipple Observatory gamma-ray telescope has been used to
observe the AGN Markarian 501 in 1996 and 1997, the years subsequent to its
initial detection. The apparent variability on the one-day time-scale observed
in TeV gamma rays in 1995 is confirmed and compared with the variability in
Markarian 421. Observations at X-ray and optical wavelengths from 1997 are also
presented.

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The categories of this paper are: hep-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: q-bio
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The categories of this paper are: quant-ph
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The categories of this paper are: eess
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The categories of this paper are: nlin
20
The categories of this paper are: acc-phys
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The categories of this paper are: cs
30
The categories of this paper are: q-alg
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The categories of this paper are: physics
12
The categories of this paper are: nucl-th
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The categories of this paper are: supr-con
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The categories of this paper are: hep-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: chem-ph
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The categories of this paper are: cmp-lg