Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Dramatic X-ray spectral variability of a Compton-thick type-1 QSO at
  $z\sim 1$   We report on the discovery of a dramatic X-ray spectral variability event
observed in a $z\sim 1$ broad line type-1 QSO. The XMM-Newton spectrum from the
year 2000 is characterized by an unobscured power-law spectrum with photon
index of $\Gamma\sim 2$, a column density of $N_{\mathrm{H}}\sim 5\times
10^{20}\,\mathrm{cm^{-2}}$, and no prominent reflection component. Five years
later, Chandra captured the source in a heavily-obscured, reflection-dominated
state. The observed X-ray spectral variability could be caused by a
Compton-thick cloud with $N_{\mathrm{H}}\sim 2\times 10^{24}\,\mathrm{cm^{-2}}$
eclipsing the direct emission of the hot corona, implying an extreme
$N_{\mathrm{H}}$ variation never before observed in a type-1 QSO. An
alternative scenario is a corona that switched off in between the observations.
In addition, both explanations require a significant change of the X-ray
luminosity prior to the obscuration or fading of the corona and/or a change of
the relative geometry of the source/reflector system. Dramatic X-ray spectral
variability of this kind could be quite common in type-1 QSOs, considering the
relatively few datasets in which such an event could have been identified. Our
analysis implies that there may be a population of type-1 QSOs which are
Compton-thick in the X-rays when observed at any given time.

16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
15
The categories of this paper are: nucl-ex
34
The categories of this paper are: chem-ph
37
The categories of this paper are: supr-con
20
The categories of this paper are: acc-phys
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
14
The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
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The categories of this paper are: hep-lat
18
The categories of this paper are: econ
22
The categories of this paper are: chao-dyn
26
The categories of this paper are: bayes-an