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Ultra-fast outflows (UFOs) are seen in many AGN, giving a possible mode forAGN feedback onto the host galaxy. However, the mechanism(s) for the launch andacceleration of these outflows are currently unknown, with UV line drivingapparently strongly disfavoured as the material along the line of sight is sohighly ionised that it has no UV transitions. We revisit this issue using theSuzaku X-ray data from PDS 456, an AGN with the most powerful UFO seen in thelocal Universe. We explore conditions in the wind by developing a new 3-DMonte-Carlo code for radiation transport. The code only handles highly ionisedions, but the data show the ionisation state of the wind is high enough thatthis is appropriate, and this restriction makes it fast enough to exploreparameter space. We reproduce the results of earlier work, confirming that themass loss rate in the wind is around 30% of the inferred inflow rate throughthe outer disc. We show for the first time that UV line driving is likely to bea major contribution to the wind acceleration. The mass loss rate in the windmatches that predicted from a purely line driven system, and this UV absorptioncan take place out of the line of sight. Continuum driving should also play arole as the source is close to Eddington. This predicts that the most extremeoutflows will be produced from the highest mass accretion rate flows onto highmass black holes, as observed.

The origin of ultra-fast outflows in AGN: Monte-Carlo simulations of the wind in PDS 456