  We address the ability of broad iron emission lines from black hole accretion
disks to diagnose the spin of the black hole. Using a high-resolution
3-dimensional MHD simulation of a geometrically-thin accretion disk in a
Pseudo-Newtonian potential, we show that both the midplane density and the
vertical column density of the accretion flow drop dramatically over a narrow
range of radii close to the innermost stable circular orbit (ISCO). We argue
that this drop of density is accompanied by a sharp increase in the ionization
parameter of the X-ray photosphere, and that the resulting imprint of the ISCO
on the X-ray reflection spectrum can be used to constrain spin. Motivated by
this simulation, we construct a simplified toy-model of the accretion flow
within the ISCO of a Kerr black hole, and use this model to estimate the
systematic error on inferred black hole spin that may result from slight
bleeding of the iron line emission to the region inside of the ISCO. We find
that these systematic errors can be significant for slowly spinning black holes
but become appreciably smaller as one considers more rapidly rotating black
holes.
