On the accuracy of mass measurement for microlensing black holes as seen
  by Gaia and OGLE
  We investigate the impact of combining Gaia astrometry from space with
precise, high cadence OGLE photometry from the ground. For the archival event
OGLE3-ULENS-PAR-02, which is likely a black hole, we simulate a realistic
astrometric time-series of Gaia measurements and combine it with the real
photometric data collected by the OGLE project. We predict that at the end of
the nominal 5 years of the Gaia mission, for the events brighter than
$G\approx15.5$ mag at the baseline, caused by objects heavier than 10
$M_{\odot}$, it will be possible to unambiguously derive masses of the lenses,
with accuracy between a few to 15 per cent. We find that fainter events
($G<17.5$) can still have their lens masses determined, provided that they are
heavier than 30 $M_{\odot}$. We estimate that the rate of astrometric
microlensing events caused by the stellar-origin black holes is $\approx 4
\times 10^{-7} \, \rm yr^{-1}$, which implies, that after 5 years of Gaia
operation and $\approx 5 \times 10^6$ bright sources in Gaia, it will be
possible to identify few such events in the Gaia final catalogues.
