The origin of ultra-fast outflows in AGN: Monte-Carlo simulations of the
  wind in PDS 456
  Ultra-fast outflows (UFOs) are seen in many AGN, giving a possible mode for
AGN feedback onto the host galaxy. However, the mechanism(s) for the launch and
acceleration of these outflows are currently unknown, with UV line driving
apparently strongly disfavoured as the material along the line of sight is so
highly ionised that it has no UV transitions. We revisit this issue using the
Suzaku X-ray data from PDS 456, an AGN with the most powerful UFO seen in the
local Universe. We explore conditions in the wind by developing a new 3-D
Monte-Carlo code for radiation transport. The code only handles highly ionised
ions, but the data show the ionisation state of the wind is high enough that
this is appropriate, and this restriction makes it fast enough to explore
parameter space. We reproduce the results of earlier work, confirming that the
mass loss rate in the wind is around 30% of the inferred inflow rate through
the outer disc. We show for the first time that UV line driving is likely to be
a major contribution to the wind acceleration. The mass loss rate in the wind
matches that predicted from a purely line driven system, and this UV absorption
can take place out of the line of sight. Continuum driving should also play a
role as the source is close to Eddington. This predicts that the most extreme
outflows will be produced from the highest mass accretion rate flows onto high
mass black holes, as observed.
