Gravitational Lenses as High-Resolution Telescopes
  The inner regions of active galaxies host the most extreme and energetic
phenomena in the universe including, relativistic jets, supermassive black hole
binaries, and recoiling supermassive black holes. However, many of these
sources cannot be resolved with direct observations. I review how strong
gravitational lensing can be used to elucidate the structures of these sources
from radio frequencies up to very high energy gamma rays. The deep
gravitational potentials surrounding galaxies act as natural gravitational
lenses. These gravitational lenses split background sources into multiple
images, each with a gravitationally-induced time delay. These time delays and
positions of lensed images depend on the source location, and thus, can be used
to infer the spatial origins of the emission. For example, using
gravitationally-induced time delays improves angular resolution of modern
gamma-ray instruments by six orders of magnitude, and provides evidence that
gamma-ray outbursts can be produced at even thousands of light years from a
supermassive black hole, and that the compact radio emission does not always
trace the position of the supermassive black hole. These findings provide
unique physical information about the central structure of active galaxies,
force us to revise our models of operating particle acceleration mechanisms,
and challenge our assumptions about the origin of compact radio emission.
Future surveys, including LSST, SKA, and Euclid, will provide observations for
hundreds of thousands of gravitationally lensed sources, which will allow us to
apply strong gravitational lensing to study the multi-wavelength structure for
large ensembles of sources. This large ensemble of gravitationally lensed
active galaxies will allow us to elucidate the physical origins of
multi-wavelength emissions, their connections to supermassive black holes, and
their cosmic evolution.
