Finding gravitational-wave black holes with parallax microlensing
  The LIGO-Virgo gravitational-wave (GW) observation unveiled the new
population of black holes (BHs) that appears to have an extended mass spectrum
up to around $70M_\odot$, much heavier than the previously-believed mass range
($\sim 8M_\odot$). In this paper, we study the capability of a microlensing
observation of stars in the Milky Way (MW) bulge region to identify BHs of GW
mass scales, taking into account the microlensing parallax characterized by the
parameter $\pi_{\rm E}\propto M^{-1/2}$ ($M$ is the mass of a lens), which is a
dimension-less quantity defined by the ratio of the astronomical unit to the
projected Einstein radius. First, assuming that BHs follow the same spatial and
velocity distributions of stars as predicted by the standard MW model, we show
that microlensing events with long light curve timescales, $t_{\rm E}\gtrsim
100~{\rm days}$, and small parallax effects, $\pi_{\rm E}\sim 10^{-2}$, are
dominated by BH lenses compared to stellar-mass lenses. Second, using a Markov
chain Monte Carlo analysis of the simulated light curve, we show that BH lens
candidates are securely identified on individual basis, if the parallax effect
is detected or well constrained to the precision of a percent level in
$\pi_{\rm E}$. We also discuss that a microlensing event of an
intermediate-mass BH of $\sim 1000M_\odot$, if it occurs, can be identified in
a distinguishable way from stellar-mass BHs.
