First detection of an isolated black hole, and mass measurement of isolated white dwarfs through astrometric microlensing

We recently detected an isolated stellar-mass black hole (BH) unambiguously for the first time, and measured its mass, distance and velocity. We used HST to carry out precise astrometry of the source star of the long-duration, high-magnification microlensing event OGLE-11-462. Combining HST astrometry, OGLE photometry, and spectroscopic measurements, we obtain a lens mass of ~7 solar mass and a distance of ~1.6 kpc. The lens emits no detectable light, which confirms its BH nature. Its proper motion is offset from the mean motion of Galactic-disk stars at similar distances by ~45 km/s, suggesting that the BH received a natal kick from its supernova explosion.

In a reprise of the famous 1919 solar-eclipse experiment that confirmed Einstein's general relativity, the nearby white dwarf Stein 2051b passed very close to a 19th magnitude background star. As it passed in front, Stein 2051B caused a deflection of the background star's image by ~2 milliarcsec, which we observed with HST. This allowed us to determine the mass of Stein 2051 B using this technique of astrometric microlensing for the first time outside the solar system. We have measured masses of 2 WDs through this technique, and our measurements lend support to the white-dwarf evolutionary theory.