A binary origin for the first isolated stellar-mass black hole detected
  with astrometric microlensing
  The Milky Way is believed to host hundreds of millions of quiescent
stellar-mass black holes (BHs). In the last decade, some of these objects have
been potentially uncovered via gravitational microlensing events. All these
detections resulted in a degeneracy between the velocity and the mass of the
lens. This degeneracy has been lifted, for the first time, with the recent
astrometric microlensing detection of OB110462. However, two independent
studies reported very different lens mass for this event. Sahu et al. (2022)
inferred a lens mass of 7.1 $\pm$ 1.3 Msun, consistent with a BH, while Lam et
al. (2022) inferred 1.6-4.2 Msun, consistent with either a neutron star or a
BH. Here, we study the landscape of isolated BHs formed in the field. In
particular, we focus on the mass and center-of-mass speed of four
sub-populations: isolated BHs from single-star origin, disrupted BHs of
binary-star origin, main-sequence stars with a compact object companion, and
double compact object mergers. Our model predicts that most ($\gtrsim$ 50%)
isolated BHs in the Milky Way are of binary origin. Moreover, the origin of
low-speed (< 50 km/s) isolated BHs depends on their mass: at least $\approx$
70% of low-mass ($\lesssim$ 10 Msun) BHs are from binary origin. Under the
assumption that OB110462 is a free-floating compact object, we conclude that it
is more likely to be a BH originally belonging to a binary system. Our results
suggest that low-speed BH microlensing events can be useful to understand
binary evolution of massive stars in the Milky Way.
