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 yr of the Gaia mission, for the events brighter than G ≈ 15.5 mag at the baseline, caused by objects heavier than 10 M_☉, it will be possible to unambiguously derive masses of the lenses, with accuracy between a few and 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_☉. We estimate that the rate of astrometric microlensing events caused by the stellar-origin black holes is ≈ 4 × 10^{- 7} yr^{- 1}, which implies, that after 5 yr of Gaia operation and ≈5 × 10⁶ bright sources in Gaia, it will be possible to identify few such events in the Gaia final catalogues.