  The fluorescent iron Ka line is a powerful observational probe of the inner
regions of black holes accretion disks. Previous studies have assumed that only
material outside the radius of marginal stability can contribute to the
observed line emission. Here, we show that fluorescence by material inside the
radius of marginal stability, which is in the process of spiralling towards the
event horizon, can have a observable influence on the iron line profile and
equivalent width. For concreteness, we consider the case of a geometrically
thin accretion disk, around a Schwarzschild black hole, in which fluorescence
is excited by an X-ray source placed at some height above the disk and on the
axis of the disk. Fully relativistic line profiles are presented for various
source heights and efficiencies. It is found that the extra line flux generally
emerges in the extreme red wing of the iron line, due to the large
gravitational redshift experienced by photons from the region within the radius
of marginal stability. We apply our models to the variable iron line seen in
the ASCA spectrum of the Seyfert nucleus MCG-6-30-15. It is found that the
change in the line profile, equivalent width, and continuum normalization, can
be well explained as being due to a change in the height of the source above
the disk. We discuss the implications of these results for distinguishing
rapidly-rotating black holes from slowly rotating holes using iron line
diagnostics.
