Abstract

It is expected that there are about 500 milions stellar black holes floating in our Galaxy. Not more than 30 are known, and only in binary systems. Even though there are some candidates for single ones, none of them was confirmed yet. Partially it comes from the fact that they are very hard to detect, but detecting multiple such objects would vastly increase our understanding of the latest stages of stellar evolution and the Galaxy structure. The perfect tool for measuring the mass of unseen objects is microlensing, yet the very special circumstances are needed to measure the mass of the lens, which is the one and only parameter which allow us to judge weather the object is a single black hole or not.

Gaia satellite will give an opportunity to look at microlensing events from the different angle. With its exquisite astrometry, it may be possible to measure not only the magnification of the source, but also its apparent displacement during the event, which provides additional information about the lens properties.

We consider a possibility of simultaneous observation of the microlensing event by the Gaia mission from space and the OGLE project from the ground. Basing on the example of OGLE -ULENS-PAR-02, which is likely a black hole, we predict that for bright events with clear parallax signal and Gaia astrometry available for them, it will be possible to determine the mass of the 10 M⊙ lens with accuracy between few to 15%. 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 years of Gaia operation and ≈ 5 × 10^6 bright sources in Gaia, it will be possible to identify few such events in its final catalogues. The talk is based on our recent paper: Rybicki et al. 2018 (2018MNRAS.476.2013R).