2018MNRAS.476.2013R

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 x 10^- 7^ yr^- 1^, which implies, that after 5 yr of Gaia operation and ≃5 x 10⁶ bright sources in Gaia, it will be possible to identify few such events in the Gaia final catalogues.