X-rays are thought to be produced in the very central regions of the accretion disk. The absorption of those X-rays by line-of-sight material is readily observable at soft X-ray energies and provides important information on the surroundings of the active nucleus. The discussion of X-ray absorption is commonly divided into that of `cold' absorption (i.e. by neutral material) and `warm' absorption (i.e. by ionized material).
Table 3: Properties of the warm absorbers in MCG-6-30-15.
The warm absorption has already been discussed in Sections 2.3.2 and 2.3.3. As previously mentioned, variability studies have led to a two-zone model for this absorber. Table 3 summarizes the properties of these two absorbing regions based on modeling with the photoionization code CLOUDY (Ferland 1991).
Once the effect of the warm absorber has been modeled, the X-ray spectrum
can be examined for evidence of additional cold absorption in excess to
that expected from our own Galaxy (the Galactic column density along the
line of sight to this source is 
).
Describing the warm absorber with a simple one-zone photoionization model
computed by the code CLOUDY (see Fabian et al. 1994 for a detailed
description of this warm absorber model), the ASCA spectrum suggests
an excess cold column density of 
. Quoted errors are statistical in nature and stated at the
90 per cent confidence level for one interesting parameter
( 
). It has been suggested that the low energy
calibration of the ASCA SIS is incorrect so as to over-estimate the
cold absorption by 1-3 
(e.g. see discussion in Cappi
et al. 1997). Thus, within these calibration uncertainties, our result may
be consistent with there being negligible cold absorption excess to the
Galactic column. Fitting the same simple model to the ROSAT PSPC
spectrum suggests an excess column density of 
, in good agreement with the ASCA result.