MHD Accretion-Disk Winds as X-ray Absorbers in AGNs
  We present the 2D ionization structure of self-similar MHD winds off
accretion disks around irradiated by a central X-ray source. Based on earlier
observational clues and theoretical arguments, we focus our attention on a
subset of these winds, namely those with radial density dependence n(r)~1/r. We
employ the photoionization code XSTAR to compute the ionic abundances of a
large number of ions of different elements and then compile their line-of-sight
(LOS) absorption columns. Particular attention is paid to the absorption
measure distribution (AMD), namely their Hydrogen-equivalent column per
logarithmic ionization parameter \xi interval, d N_H/(d \log \xi), which
provides a measure of the winds' radial density profiles. For n(r)~1/r the AMD
is found to be independent of \xi, in good agreement with its behavior inferred
from the X-ray spectra of several AGNs. For the specific wind structure and
X-ray spectrum we also compute detailed absorption line profiles for a number
of ions to obtain their LOS velocities, v~100-300 km/sec (at \log \xi~2-3) for
Fe XVII and v~1,000-4,000 km/sec (at \log \xi~4-5) for Fe XXV, in good
agreement with the observation. Our models describe the X-ray absorption
properties of these winds with only two parameters, namely the mass-accretion
rate \dot{m} and LOS angle \theta. The probability of obscuration of the X-ray
ionizing source in these winds decreases with increasing \dot{m} and increases
steeply with \theta. As such, we concur with previous authors that these wind
configurations, viewed globally, incorporate all the requisite properties of
the parsec scale "torii" invoked in AGN unification schemes. We indicate that a
combination of the AMD and absorption line profile observations can uniquely
determine these model parameters and their bearing on AGN population
demographics.
