Effects of Kerr Strong Gravity on Quasar X-ray Microlensing
  Recent quasar microlensing observations have constrained the sizes of X-ray
emission regions to be within about 10 gravitational radii of the central
supermassive black hole. Therefore, the X-ray emission from lensed quasars is
first strongly lensed by the black hole before it is lensed by the foreground
galaxy and star fields. We present a scheme that combines the initial strong
lensing of a Kerr black hole with standard linearized microlensing by
intervening stars. We find that X-ray microlensed light curves incorporating
Kerr strong gravity can differ significantly from standard curves. The
amplitude of the fluctuations in the light curves can increase or decrease by
~0.65-0.75 mag by including Kerr strong gravity. Larger inclination angles give
larger amplitude fluctuations in the microlensing light curves. Consequently,
current X-ray microlensing observations might have under or overestimated the
sizes of the X-ray emission regions. We estimate this bias using a simple
metric based on the amplitude of magnitude fluctuations. The half light radius
of the X-ray emission region can be underestimated up to ~50% or overestimated
up to ~20%. Underestimates are found in most situations we have investigated.
The only exception is for a disk with large spin, radially flat emission
profile, and observed nearly face on, where an overestimate is found. Thus,
more accurate microlensing size constraints should be obtainable by including
Kerr lensing. The caustic crossing time can differ by months after including
Kerr strong gravity. A simultaneous monitoring of gravitational lensed quasars
in both X-ray and optical bands with densely sampled X-ray light curves might
reveal this feature. We conclude that it should be possible to constrain
important parameters such as inclination angles and black hole spins from
combined Kerr and microlensing effects.
