Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ubiquitous variability of X-ray absorbing column densities in Seyfert 2
  Galaxies   We present a study of the variations in the absorbing column density of 25
X-ray defined Seyfert 2 galaxies, as inferred from hard X-ray observations, on
timescales from months to several years. We show that a significant variation
of N$_H$ (from 20% to 80%) is observed in almost all (22/25) the sources with
multiple X-ray observations, although X-ray absorption never vanishes. For a
subsample of 11 sources observed at least five times the typical variation
time, as defined by a structure function, is less than one year for both
heavily absorbed (nH ~ 10^23 cm^(-2)) and moderately absorbed (nH ~ 10^22
cm^(-2)) sources. These variations rule out the simplest version of the unified
models, based on a homogeneous obscuring torus, and suggest the presence of
clumpy circumnuclear material on a scale well below a parsec. We propose a
modification of the torus model in which an overabundance of slightly dusty
BELR clouds obscures the BELR. The BELR needs, like the torus, to have an
axisymmetric structure. This model is closely related to that of Elvis (2000)
for type 1 AGN. For lightly obscured AGN (nH ~ 10^22 cm^(-2)) the structure
function shows an increase at a timescale of $\sim$5 yr, indicating a second
absorber, most probably on a 5-10 pc scale associated with the host galaxy.

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The categories of this paper are: alg-geom
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The categories of this paper are: mtrl-th
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The categories of this paper are: supr-con
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The categories of this paper are: patt-sol
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The categories of this paper are: dg-ga
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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: nucl-ex
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The categories of this paper are: stat
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The categories of this paper are: econ
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The categories of this paper are: adap-org
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The categories of this paper are: cond-mat
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The categories of this paper are: nucl-th
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The categories of this paper are: physics
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The categories of this paper are: solv-int
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The categories of this paper are: acc-phys