Resolving the Inner Structure of QSO Discs by Fold Caustic Crossing
  Events
  Though the bulk of the observed optical flux from the discs of
intermediate-redshift lensed quasars is formed well outside the region of
strong relativistic boosting and light-bending, relativistic effects have
important influence on microlensing curves. The reason is in the divergent
nature of amplification factors near fold caustics increasingly sensitive to
small spatial size details. Higher-order disc images produced by strong light
bending around the black hole may affect the amplification curves, making a
contribution of up to several percent near maximum amplification. In accordance
with theoretical predictions, some of the observed high-amplification events
possess fine structure. Here we consider three putative caustic crossing
events, one by SBS1520+530 and two events for individual images of the
Einstein's cross (QSO J2237+0305). Using relativistic disc models allows to
improve the fits, but the required inclinations are high, about 70deg or
larger. Such high inclinations apparently contradict the absence of any strong
absorption that is likely to arise if a disc is observed edge-on through a dust
torus. Still, the high inclinations are required only for the central parts of
the disc, that allows the disc itself to be initially tilted by 60-90deg with
respect to the black hole and aligned toward the black hole equatorial plane
near the last stable orbit radius. For SBS1520+530, an alternative explanation
for the observed amplification curve is a superposition of two subsequent fold
caustic crossings. While relativistic disc models favour black hole masses
~10^10 solar (several times higher than the virial estimates) or small
Eddington ratios, this model is consistent with the observed distribution of
galaxies over peculiar velocities only if the black hole mass is about 3 10^8
solar.
