X-Ray Line Profile Variations during Quasar Microlensing
  Observations of several gravitationally microlensed quasars in X-rays
revealed variations in the profile of the iron K$\alpha$ line in the course of
microlensing events. We explore the effect by simulating a microlensing caustic
crossing a spatially resolved model of emission from a thin accretion disk
around a Kerr black hole. We demonstrate the sequence of spectral changes
during the event, in particular the appearance of additional peaks and edges in
the line profile due to microlensing. We trace the origin of these features to
points on the disk, at which the total energy shift ($g$-factor) contours are
tangent to the caustic. Contours tangent from the inner side of the caustic
generate peaks, while those tangent from its outer side generate edges. We
derive analytical shapes of the generated features and map the peak strength as
a function of position of the tangent point on the disk. Since the features are
determined by the positional geometry of the caustic relative to the $g$-factor
contours, the same type of behavior can be expected in a much broader range of
emission models. The sequence of line profile changes thus serves as a
sensitive probe of the geometry and physics of the innermost region of the
quasar accretion disk.
