MNRAS, in press. astro-ph/0303143
A. Merloni (1), A. C. Fabian (2)
1) Max-Planck-Institut fuer
Astrophysik, K-Swarzschild Str., 1, 85741, Garching, Germany
2) Institute of Astronomy,
Cambridge, CB3 0HA, UK
Abstract
Recent X-ray observations have shown evidence for exceptionally
broad and skewed iron K$\alpha$ emission lines from several accreting
black hole systems. The lines are assumed to be due to fluorescence of
the accretion disk illuminated
by a surrounding corona and require a steep emissivity profile
increasing in to the innermost radius. This appears to
question both standard accretion disc theory and the zero torque
assumption for the inner boundary condition, both of which predict a
much less extreme profile. Instead it argues that a torque may be
present due to magnetic coupling with matter in the plunging region or
even to the spinning black hole itself. Discussion so far has centered
on the torque acting on the disc. However the crucial determinant of
the iron line profile is the radial variation of the power radiated in
the corona. Here we study the effects of different inner boundary
conditions on the coronal emissivity and on the profiles of the
observable Fe K$\alpha$ lines.
We argue that in the extreme case where a prominent high redshift
component of the iron line is
detected, requiring a steep emissivity profile in the innermost part
and a flatter one outside, energy from the gas
plunging into the black hole is being fed directly to the corona.