PopSyCLE: A New Population Synthesis Code for Compact Object
  Microlensing Events
  We present a new Milky Way microlensing simulation code, dubbed PopSyCLE
(Population Synthesis for Compact object Lensing Events). PopSyCLE is the first
resolved microlensing simulation to include a compact object distribution
derived from numerical supernovae explosion models and both astrometric and
photometric microlensing effects. We demonstrate the capabilities of PopSyCLE
by investigating the optimal way to find black holes (BHs) with microlensing.
Candidate BHs have typically been selected from wide-field photometric
microlensing surveys, such as OGLE, by selecting events with long Einstein
crossing times ($t_E>120$ days). These events can be selected at closest
approach and monitored astrometrically in order to constrain the mass of each
lens; PopSyCLE predicts a BH detection rate of ~40% for such a program. We find
that the detection rate can be enhanced to ~85% by selecting events with both
$t_E>120$ days and a microlensing parallax of $\pi_E<0.08$. Unfortunately, such
a selection criterion cannot be applied during the event as $\pi_E$ requires
both pre- and post-peak photometry. However, historical microlensing events
from photometric surveys can be revisited using this new selection criteria in
order to statistically constrain the abundance of BHs in the Milky Way. The
future WFIRST microlensing survey provides both precise photometry and
astrometry and will yield individual masses of $\mathcal{O}(100-1000) black
holes, which is at least an order of magnitude more than is possible with
individual candidate follow-up with current facilities. The resulting sample of
BH masses from WFIRST will begin to constrain the shape of the black hole
present-day mass function, BH multiplicity, and BH kick velocity distributions.
