Constraints on Compton-thick winds from black hole accretion disks: can we see the inner disk?

TL;DR

This paper argues that sub-Eddington black hole accretion disks cannot produce Compton-thick winds that obscure the inner disk, implying we can observe the inner regions directly in such systems.

Contribution

It demonstrates that known wind-driving mechanisms are insufficient to generate Compton-thick winds in sub-Eddington black hole systems, supporting the idea that the inner disk remains visible.

Findings

Compton-thick winds require unphysically high momentum exceeding photon momentum.

Radiative acceleration is ineffective unless near Eddington luminosity.

Thermal driving is ruled out due to large Compton radii.

Abstract

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.