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Date and Time of the Query: 2019-07-23 T05:33:02 PDT
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For refcode 2002ApJ...571..234R:
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2002ApJ...571..234R Ubiquitous Variability of X-Ray-absorbing Column Densities in Seyfert 2 Galaxies G. Risaliti Osservatorio Astrofisico di Arceti, Largo Enrico Fermi 5, I-50125 Firenze, Italy; grisaliti@cfa.harvard.edu and M. Elvis and F. Nicastro Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Received 2001 April 19; accepted 2001 August 29 ABSTRACT 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 of 25) of 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 1 yr for both heavily absorbed (N_H_ ~ 10^23^ cm^-2^) and moderately absorbed (N_H_ ~ 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 broad emission-line region (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 for type 1 active galactic nuclei (AGNs). For lightly obscured AGNs (N_H_ ~ 10^22^ cm^-2^), the structure function shows an increase at a timescale of ~5 yr, indicating a second absorber, most probably on a 5-10 pc scale associated with the host galaxy. Subject headings: galaxies: active - galaxies: Seyfert - X-rays: galaxies
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