An Isolated Stellar-Mass Black Hole Detected Through Astrometric
  Microlensing
  We report the first unambiguous detection and mass measurement of an isolated
stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry
out precise astrometry of the source star of the long-duration (t_E ~ 270
days), high-magnification microlensing event
MOA-2011-BLG-191/OGLE-2011-BLG-0462, in the direction of the Galactic bulge.
HST imaging, conducted at eight epochs over an interval of six years, reveals a
clear relativistic astrometric deflection of the background star's apparent
position. Ground-based photometry shows a parallactic signature of the effect
of the Earth's motion on the microlensing light curve. Combining the HST
astrometry with the ground-based light curve and the derived parallax, we
obtain a lens mass of 7.1 +/- 1.3 M_Sun and a distance of 1.58 +/- 0.18 kpc. We
show that the lens emits no detectable light, which, along with having a mass
higher than is possible for a white dwarf or neutron star, confirms its BH
nature. Our analysis also provides an absolute proper motion for the BH. The
proper motion is offset from the mean motion of Galactic-disk stars at similar
distances by an amount corresponding to a transverse space velocity of ~45
km/s, suggesting that the BH received a modest natal 'kick' from its supernova
explosion. Previous mass determinations for stellar-mass BHs have come from
radial-velocity measurements of Galactic X-ray binaries, and from gravitational
radiation emitted by merging BHs in binary systems in external galaxies. Our
mass measurement is the first ever for an isolated stellar-mass BH using any
technique.
