Physical Conditions in Ultra Fast Outflows in AGN
  XMM-Newton and Suzaku spectra of AGN have revealed highly ionized gas, in the
form of absorption lines from H-like and He-like Fe. Some of these absorbers,
"Ultra Fast Outflows (UFOs)", have radial velocities of up to 0.25c. We have
undertaken a detailed photo-ionization study of high-ionization Fe absorbers,
both UFOs and non-UFOs, in a sample of AGN observed by XMM-Newton. We find that
the heating and cooling processes in UFOs are Compton-dominated, unlike the
non-UFOs. Both types are characterized by Force Multipliers on the order of
unity, which suggests that they cannot be radiatively accelerated in
sub-Eddington AGN, unless they were much less ionized at their point of origin.
However, such highly ionized gas can be accelerated via a Magneto-Hydrodynamic
(MHD) wind. We explore this possibility by applying a cold MHD flow model to
the UFO in the well-studied Seyfert galaxy, NGC 4151. We find that the UFO can
be accelerated along magnetic streamlines anchored in the accretion disk. In
the process, we have been able to constrain the magnetic field strength and the
magnetic pressure in the UFO and have determined that the system is not in
magnetic/gravitational equipartition.Open questions include the variability of
the UFOs and the apparent lack of non-UFOs in UFO sources.
