Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The WEBT campaign to observe AO 0235+16 in the 2003-2004 observing
  season   A multiwavelength campaign on the BL Lac object AO 0235+16 has been organized
by the WEBT collaboration during the observing seasons 2003-2004 and 2004-2005.
Here we report on the results of the first season, which saw the participation
of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the
first XMM-Newton pointing, which occurred on January 18-19, 2004. Unpublished
data from previous epochs were also collected. In the optical and near-IR the
source has been very active in the last 3 years, though rather faint most of
the time, with noticeable variations of more than a mag in a few days. In
contrast, in the radio bands it appears "quiescent" since early 2000. The X-ray
spectra obtained by the three EPIC detectors are well fitted by a power law
with extra-absorption at z=0.524. No significant variation of the X-ray flux
occurred during the pointing. In contrast, contemporaneous dense radio
monitoring with the 100 m telescope at Effelsberg shows a ~2-3% flux decrease
in 6-7 hours, which, if intrinsic, would imply a brightness temperature well
above the Compton limit. We construct the broad-band spectral energy
distribution of January 18-19, 2004 with simultaneous radio data from
Effelsberg, optical data from the Nordic Optical Telescope, optical-UV data
from the Optical Monitor onboard XMM-Newton, and X-ray data by the EPIC
instruments. Particular care was taken to correct data for extinction due to
both the Milky Way and the z=0.524 absorber. The resulting SED suggests the
existence of a bump in the UV spectral region.

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The categories of this paper are: mtrl-th
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The categories of this paper are: math-ph
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The categories of this paper are: hep-th
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The categories of this paper are: nlin
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The categories of this paper are: chem-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-lat
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The categories of this paper are: quant-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: stat
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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: cs
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The categories of this paper are: q-bio
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The categories of this paper are: chao-dyn