Astro2020 Science White Paper: A New Era for X-ray Lensing Studies of
  Quasars and Galaxies
  Current X-ray observations and simulations show that gravitational lensing
can be used to infer the structure near the event horizons of black holes,
constrain the dynamics and evolution of black-hole accretion and outflows, test
general relativity in the strong-gravity regime and place constraints on the
evolution of dark matter in the lensing galaxies. These science goals currently
cannot be achieved in a statistically large sample of z = 0.5 - 5 lensed
quasars due to the limited capabilities of current X-ray telescopes and the
relatively low number (~200) of known lensed quasars. The latter limitation
will be resolved with the multi-band and wide-field photometric optical survey
of LSST that is expected to lead to the discovery of > 4,000 additional
gravitationally lensed systems. As we show in this white paper, these science
goals can be reached with an X-ray telescope having a spatial resolution of
<0.5arcsec to resolve the lensed images and a collecting area of >0.5 m^2 at 1
keV.
