Location, size and geometry of the absorbing medium have been some of the most debated topics. Although somewhat artificially, the gaseous, X-ray absorber can be roughly divided in two main components: an extended medium (on scales of 10 - 100 pc) and a compact, nuclear absorber (< 1 - 10 pc).
Evidence for an absorbing medium on scales of about 100 pc, possibly associated with the host galaxy gaseous disk, has been inferred from the statistical properties of the hosts and of the NLR of Seyfert galaxies (e.g. [Maiolino & Rieke(1995)]), from the direct observation of obscuring structures in high resolution images (e.g. [Malkan et al. 1998]) and from the detection of circumnuclear molecular gas (e.g. [Schinnerer et al. (1999)]). The X-ray absorption properties of such large scale absorbers have been studied with some detail by [Guainazzi et al. (2005)]. They found that Seyfert nuclei with dusty structures, observed within the central few 100 pc in HST images, are typically characterized by an absorbed, but Compton thin X-ray spectrum.
Evidence for an additional, much more compact gaseous absorber is directly obtained from X-ray data. There are two lines of evidence, one based on dynamical mass constraints and the other one on time variability.
[Risaliti et al. (1999)] showed that, for any reasonable geometry, gas with NH > 1024 cm-2 cannot be accommodated on scales larger than a few 10 pc without exceeding the dynamical mass in the same region. Therefore, the Compton thick medium must be located within the central ~ 10 pc.
Tighter constraints come from the observed temporal variability of the absorbing column density. [Risaliti et al. (2002)] showed that variability of the X-ray absorbing column density on time scales of years is observed in nearly all AGNs for which multi-epoch X-ray observations are available. For a subsample of the sources variability is observed even on scale shorter than one year. For any reasonable geometry of the individual clouds, the latter result implies that in these sources most of the the X-ray absorption must occur on sub-parsec scales. Even tighter constraints come from dedicated X-ray monitoring of some individual sources [Risaliti et al. (2005), Elvis et al. (2004)]. These observations revealed strong NH variations, by even passing from the Compton thin to the Compton thick regime, on time scales of weeks or even as short as a few hours (Fig. 2). These rapid variations indicate that the absorber must much closer to the source than the standard pc-scale model, and probably co-spatial with the Broad Line Region. Further results and details on the variability of X-ray absorption are given in the contribution by Risaliti within these conference proceedings.
Figure 2. Unfolded spectra of NGC 1365 taken during a period of about 8 months showing strong NH variations (from [Risaliti et al. (2005)]). The most rapid variation is observed between observations XMM1 and XMM2, which are only three weeks apart, and which show a transition from Compton thin to Compton thick.