Revealing the Location and Structure of the Accretion Disk Wind in PDS 456

Abstract

We present evidence for the rapid variability of the high-velocity iron K-shell absorption in the nearby (z = 0.184) quasar PDS 456. From a recent long Suzaku observation in 2013 (~1 Ms effective duration), we find that the equivalent width of iron K absorption increases by a factor of ~5 during the observation, increasing from <105 eV within the first 100 ks of the observation, toward a maximum depth of ~500 eV near the end. The implied outflow velocity of ~0.25 c is consistent with that claimed from earlier (2007, 2011) Suzaku observations. The absorption varies on timescales as short as ~1 week. We show that this variability can be equally well attributed to either (1) an increase in column density, plausibly associated with a clumpy time-variable outflow, or (2) the decreasing ionization of a smooth homogeneous outflow which is in photo-ionization equilibrium with the local photon field. The variability allows a direct measure of absorber location, which is constrained to within r = 200-3500 r g of the black hole. Even in the most conservative case, the kinetic power of the outflow is gsim 6% of the Eddington luminosity, with a mass outflow rate in excess of ~40% of the Eddington accretion rate. The wind momentum rate is directly equivalent to the Eddington momentum rate which suggests that the flow may have been accelerated by continuum scattering during an episode of Eddington-limited accretion.