Constraints on Compton-Thick Winds From Black Hole Accretion Disks: Can We See the Inner Disk?

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 Kα 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/λ, where λ 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.

Ultrafast outflows in radio-loud active galactic nuclei

“…The typical location of the UFOs is estimated to be in the range between ∼10 2 -10 4 rs (rs=GMBH /c 2 ) from the central SMBH, indicating a direct association with winds originating from the accretion disk (e.g., Tombesi et al 2012bTombesi et al , 2013b. Their high ionization levels and velocities are, to date, consistent with both radiation pressure and magnetohydrodynamic (MHD) processes contributing to the outflow acceleration (e.g., Everett & Ballantyne 2004;Everett 2005;Ohsuga et al 2009;King 2010;Fukumura et al 2010Fukumura et al , 2014Kazanas et al 2012;Ramirez & Tombesi 2012;Reynolds 2012;Tombesi et al 2013b). The study of UFOs in radio-loud AGNs can provide important insights into the characteristics of the inner regions of these sources and can provide an important test for theoretical models trying to explain the disk-jet connection (e.g., McKinney 2006;Tchekhovskoy et al 2011;Yuan, Bu & Wu 2012;Sadowski et al 2013Sadowski et al , 2014.…”

Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion

“…The iron abundance in the surface disk atmosphere of a torus-like geometry is expected to be higher than in the accretion flow as a whole. This is because certain ions are preferentially dragged upward owing to their magnetic moments, similarly to what happens in the solar corona where abundances differ from the solar photosphere (Reynolds 2012). Since no other spectra show evidence for a line beside the strongest detected line (that should be associated with the strongest line detected by Chandra, therefore the iron-Kα emission line), this result suggests that the ionization level should be lower in all the remaining Swift/XRT spectra.…”

Observing black holes spin

“…X-ray reflection from more distant gaseous structures as well as X-ray absorption from material in the Milky Way, the host galaxy of the AGN, and any accretion disk wind must be carefully modeled and accounted for before the subtle effects of spin can be teased from the data. In particular, the degeneracy of spin signatures with those of accretion disk winds has sparked lively debate [20,21,22]. Recent work has also explored systematic errors in spin measurements that result from the finite thickness of the accretion disk [23], finding them to become increasingly relevant as one considers black holes with higher accretion rates.…”