Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Hellas2XMM survey: III. Multiwavelength observations of hard X-ray
  selected sources in the PKS 0312-77 field   We present extensive optical, radio and infrared follow-up observations of a
sample of 35 hard X-ray (2-10 keV) selected sources discovered serendipitously
in the PV XMM-Newton observation of the radio-loud quasar PKS 0312-77 field,
for which also an archival Chandra observation is available. The observations
have been carried out as part of the HELLAS2XMM survey, a program aimed to
understand the nature of the sources responsible for the bulk of the hard X-ray
Background (XRB). The identification of the optical counterparts greatly
benefits from the positional accuracy obtained from Chandra and radio
observations. As a consequence, the spectroscopic completeness of the present
sample (80%) is limited only by the faintness of the optical counterparts. The
multiwavelength coverage of our survey allows us to unveil a large spread in
the overall properties of hard X-ray selected sources. At low redshift (z<1),
the source breakdown includes Broad Line AGN, Narrow Emission-Line Galaxies,
and optically ``normal'' galaxies. All the ten sources at z>1 are
spectroscopically classified as Broad Line AGNs. A few of them show significant
intrinsic X-ray absorption (N(H)>10^22 cm^-2), further supporting previous
evidence of a decoupling between optical and X-ray properties at high
luminosities and redshifts. Finally, a non negligible fraction (15%) of the
hard X-ray sources are not detected down to the limiting magnitude of the
optical images. The corresponding high X-ray to optical flux ratio, X-ray and
optical-infrared colors strongly suggest that they are high redshift, obscured
AGN.

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The categories of this paper are: acc-phys
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The categories of this paper are: gr-qc
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The categories of this paper are: atom-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: hep-ph
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The categories of this paper are: q-bio
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The categories of this paper are: chem-ph
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The categories of this paper are: hep-th
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The categories of this paper are: adap-org
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The categories of this paper are: cmp-lg
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The categories of this paper are: patt-sol
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The categories of this paper are: math-ph
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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: nucl-th