  Accretion around black holes is very often characterized by distinctive X-ray
reflection features (mostly, iron inner-shell transitions), which arise due to
the primary radiation being reprocessed by a dense and relatively colder
medium, such as an accretion disk. Most reflection modeling assume that
emission stops at the inner-most stable circular orbit (ISCO), and that for
smaller radii - in the plunging region - the density drops and the accretion
flow is far too ionized for efficient line production. We investigate the
spectral features of the reflection in the plunging regions of optically-thick
and geometrically-thin accretion disks around black holes. We show that for
cases in which the density profile is considered constant (as expected in
highly magnetized flows), or in cases in which the disk density is high enough
such that the ionization still allows line formation within the ISCO, there is
a significant modification of the observed reflected spectrum. Consistent with
previous studies, we found that the impact of the radiation reprocessed in the
plunging region is stronger the lower the black hole spin, when the plunging
region subtends a larger area. Likewise, as for the case of standard reflection
modeling, the relativistic broadening of the iron line is more pronounced at
low inclination, whereas the blueshift and relativistic beaming effect is
dominant at high inclination. We also tested the effects of various
prescriptions of the stress at the ISCO radius on the reflection spectrum, and
found that several of these cases appear to show line profiles distinct enough
to be distinguishable with reasonably good quality observational data.
