Astro2020 Science White Paper - Quasar Microlensing: Revolutionizing our
  Understanding of Quasar Structure and Dynamics
  Microlensing by stars within distant galaxies acting as strong gravitational
lenses of multiply-imaged quasars, provides a unique and direct measurement of
the internal structure of the lensed quasar on nano-arcsecond scales. The
measurement relies on the temporal variation of high-magnification caustic
crossings which vary on timescales of days to years. Multiwavelength
observations provide information from distinct emission regions in the quasar.
Through monitoring of these strong gravitational lenses, a full tomographic
view can emerge with Astronomical-Unit scale resolution. Work to date has
demonstrated the potential of this technique in about a dozen systems. In the
2020s there will be orders of magnitude more systems to work with. Monitoring
of lens systems for caustic-crossing events to enable triggering of
multi-platform, multi-wavelength observations in the 2020s will fulfill the
potential of quasar microlensing as a unique and comprehensive probe of active
black hole structure and dynamics.
