Constraints on Compton-thick winds from black hole accretion disks: can we see the inner disk?. (arXiv:1210.3029v1 [astro-ph.HE])

Constraints on Compton-thick winds from black hole accretion disks: can we see the inner disk?. (arXiv:1210.3029v1 [astro-ph.HE]):
Strong evidence is emerging that winds can be driven from the central regions
of accretion disks in both active galactic nuclei (AGN) and Galactic black hole
binaries (GBHBs). Direct evidence for highly-ionized, Compton-thin inner-disk
winds comes from observations of blueshifted (v~0.05-0.1c) iron-K X-ray
absorption lines. However, it has been suggested that the inner regions of
black hole accretion disks can also drive Compton-thick winds --- such winds
would enshroud the inner disk, preventing us from seeing direct signatures of
the accretion disk (i.e. the photospheric thermal emission, or the
Doppler/gravitationally broadened iron K-alpha line). Here, we show that,
provided the source is sub-Eddington, the well-established wind driving
mechanisms fail to launch a Compton-thick wind from the inner disk. For the
accelerated region of the wind to be Compton-thick, the momentum carried in the
wind must exceed the available photon momentum by a factor of at least
2/lambda, where lambda is the Eddington ratio of the source, ruling out
radiative acceleration unless the source is very close to the Eddington limit.
Compton-thick winds also carry large mass-fluxes, and a consideration of the
connections between the wind and the disk show this to be incompatible with
magneto-centrifugal driving. Finally, thermal driving of the wind is ruled out
on the basis of the large Compton-radii that typify black hole systems. In the
absence of some new acceleration mechanism, we conclude that the inner regions
of sub-Eddington accretion disks around black holes are indeed naked.