On the accuracy of mass measurement for microlensing black holes as seen by Gaia and OGLE

TL;DR

Combining Gaia astrometry with OGLE photometry enables precise mass measurements of black hole microlensing events, potentially identifying a few stellar-origin black holes over five years.

Contribution

This study demonstrates that Gaia astrometry combined with OGLE photometry can accurately determine masses of black hole lenses, improving detection prospects.

Findings

Masses of lenses can be measured with 5-15% accuracy for bright events.

Fainter events (>17.5 mag) can still have mass estimates if >30 M_sun.

Estimated black hole microlensing rate is about 4 x 10^-7 per year.

Abstract

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.