Iron K\alpha line profiles and the inner boundary condition of accretion flows

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


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.