Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Energy distribution of individual quasars from far-UV to X-rays: I.
  Intrinsic UV hardness and dust opacities   Using Chandra and HST archival data, we have studied the individual Spectral
Energy Distribution (SED) of 11 quasars. All UV spectra show a spectral break
around 1100A. 5 X-ray spectra show the presence of a ``soft excess'' and 7
spectra showed an intrinsic absorption. We found that for most quasars a simple
extrapolation of the far-UV powerlaw into the X-ray domain generally lies below
the X-ray data and that the big blue bump and the soft X-ray excess do not
share a common physical origin. We explore the issue of whether the observed
SED might be dust absorbed in the far and near-UV. We fit the UV break,
assuming a powerlaw that is absorbed by cubic nanodiamond dust grains. We then
explore the possibility of a universal SED (with a unique spectral index) by
including further absorption from SMC-like extinction. Using this approach,
satisfactory fits to the spectra can be obtained. The hydrogen column densities
required by either nanodiamonds or amorphous dust models are all consistent,
except for one object, with the columns deduced by our X-ray analysis, provided
that the C depletion is ~0.6. Because dust absorption implies a flux recovery
in the extreme UV (<700A), our modeling opens the possibility that the
intrinsic quasar SED is much harder and more luminous in the extreme UV than
inferred from the near-UV data, as required by photoionization models of the
broad emission line region. We conclude that the intrinsic UV SED must undergo
a sharp turn-over before the X-ray domain.

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The categories of this paper are: gr-qc
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The categories of this paper are: patt-sol
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The categories of this paper are: math
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The categories of this paper are: physics
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The categories of this paper are: q-bio
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The categories of this paper are: nlin
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The categories of this paper are: cmp-lg
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The categories of this paper are: alg-geom
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The categories of this paper are: chem-ph
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The categories of this paper are: adap-org
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The categories of this paper are: acc-phys
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The categories of this paper are: solv-int
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-alg
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The categories of this paper are: ao-sci
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The categories of this paper are: cond-mat