Dark matter under the microscope: Constraining compact dark matter with
  caustic crossing events
  A galaxy cluster acts as a cosmic telescope over background galaxies but also
as a cosmic microscope of the lens imperfections. The diverging magnification
of lensing caustics enhances the microlensing effect of substructure present
within the lensing mass. Fine-scale structure can be accessed as a moving
background source brightens and disappears when crossing these caustics. The
recent recognition of a distant lensed star near the Einstein radius of the
galaxy cluster MACSJ1149.5+2223 (Kelly et al. 2017) allows the rare opportunity
to reach subsolar mass microlensing through a super-critical column of cluster
matter. Here we compare these observations with high-resolution ray-tracing
simulations that include stellar microlensing set by the observed intracluster
starlight and also primordial black holes that may be responsible for the
recently observed LIGO events. We explore different scenarios with microlenses
from the intracluster medium and black holes, including primordial ones, and
examine strategies to exploit these unique alignments. We find that the best
constraints on the fraction of compact dark matter in the small-mass regime can
be obtained in regions of the cluster where the intracluster medium plays a
negligible role. This new lensing phenomenon should be widespread and can be
detected within modest-redshift lensed galaxies so that the luminosity distance
is not prohibitive for detecting individual magnified stars. Continuous {\it
Hubble Space Telescope} monitoring of several such optimal arcs will be
rewarded by an unprecedented mass spectrum of compact objects that can
contribute to uncovering the nature of dark matter.
