Extreme-Velocity Quasar Outflows and the Role of X-ray Shielding
  Quasar accretion disk winds observed via broad absorption lines (BALs) in the
UV produce strong continuous absorption in X-rays. The X-ray absorber is
believed to serve critically as a radiative shield to enable radiative driving.
However, "mini-BAL" and narrow absorption line outflows have dramatically less
X-ray absorption than BALs. Here we examine X-ray and rest-frame UV spectra of
8 mini-BAL quasars with outflow speeds in the range 0.1c to 0.2c to test
whether extreme speeds require a strong shield. We find that the X-ray
absorption is weak or moderate, with neutral-equivalent column densities N_H <
few times 10^22 cm^-2, consistent with mini-BALs at lower speeds. We use
photoionization models to show that this amount of shielding is too weak to
control the outflow ionizations and, therefore, it is not important for the
acceleration. Shielding in complex geometries also seems unlikely because the
alleged shield would need to extinguish the ionizing far-UV flux while avoiding
detection in X-rays and the near-UV.
  We argue that the outflow ionizations are kept moderate, instead, by high gas
densities in small clouds. If the mini-BALs form at radial distances of order R
~ 2 pc from the central quasar (broadly consistent with theoretical models and
with the mini-BAL variabilities observed here and in previous work), and the
total column densities in the mini-BAL gas are N_H ~< 10^21 cm^-2, then the
total radial extent of outflow clouds is only Delta-R_clouds ~< 3 x 10^13 cm
and the radial filling factor is Delta-R_clouds/R ~< 5 x 10^-6 for
weak/negligible shielding. Compared to the transverse sizes >~ 8 x 10^15 cm
(based on measured line depths), the outflows have shapes like thin "pancakes"
viewed face-on, or they occupy larger volumes like a spray of many dense clouds
with a small volume filling factor. These results favor models with magnetic
confinement in magnetic disk winds.
