General considerations

Finally, we discuss the implications of these results for theoretical models of the central engine. In this discussion, we shall assume that the hard-band X-ray spectral features arise from X-ray illumination of a flat accretion disk orbiting a rapidly-rotating black hole in the prograde sense in the $\theta=\pi/2$ plane. We shall also assume that the steep emissivity profile of the inner disk is due to a violation of the standard zero-torque boundary condition at $r=r_{\rm ms}$.

We must note that the physics of the inner disk boundary is still very uncertain and the subject of current work and debate. The motivation behind the TORQUED model of Section 3.4 was the presence of a magnetic connection between the inner disk and either the plunging region (Agol & Krolik 2000) or the rotating (stretched) event horizon itself (Li 2002). However, all aspects of this scenario have been challenged and debated. Li (2003) analyzed the structure of the magnetic field within the plunging region and argued that the magnetic connection is too weak for the plunging region to influence the rest of the disk. However, these arguments are tempered by the fact that it seems to be rather easy to torque the disk with the plunging regions in simulated accretion disks (e.g., Hawley & Krolik 2001; Reynolds & Armitage 2002). On a different note, Merloni & Fabian (2003) have used the Merloni (2003) model for the energization of the disk corona to argue that the inner corona is strongly suppressed by disk torquing. In other words, while the dissipation profile in a torqued disk can be very centrally concentrated, it might be hard to translate this into a centrally concentrated X-ray emission pattern. Instead, they suggest that magnetic connections with the plunging region or rotating black hole energize the corona directly. A possible problem with this scenario is the requirement that the corona can transport the angular momentum released by the plunging region or black hole. Finally, Williams (2003) has challenged the notion that magnetic fields are relevant for energizing the innermost disk. She shows that Penrose scattering processes (Penrose 1969; Williams 1995) can lead to a non-magnetic spin energy extraction mechanism.

It is beyond the scope of this (observational) paper to address these physical processes in any detail. For now, we loosely refer to all of the above models and variants as ``torqued disk models'', and assume that some form of interaction with the plunging region or ergosphere of the black hole is energizing the inner disk/corona and producing the steep emissivity profile seen in the observations.