An isolated mass gap black hole or neutron star detected with
  astrometric microlensing
  We present the analysis of five black hole (BH) candidates identified from
gravitational microlensing surveys. HST astrometric data and densely sampled
lightcurves from ground-based microlensing surveys are fit with a
single-source, single-lens microlensing model in order to measure the mass and
luminosity of each lens and determine if it is a black hole. One of the five
targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows a
significant $>1$ mas coherent astrometric shift, little to no lens flux, and
has an inferred lens mass of 1.6 - 4.2 $M_\odot$. This makes OB110462 the first
definitive discovery of a compact object through astrometric microlensing and
it is most likely either a neutron star or a low-mass black hole. This compact
object lens is relatively nearby (690 - 1370 pc) and has a slow transverse
motion of $<$ 25 km/s. OB110462 shows significant tension between models
well-fit to photometry vs. astrometry, making it currently difficult to
distinguish between a neutron star and a black hole. Additional observations
and modeling with more complex system geometries, such as binary sources are
needed to resolve the puzzling nature of this object. For the remaining four
candidates, the lens masses are $<2 M_\odot$ and they are unlikely to be black
holes; but two of the four are likely white dwarfs or neutron stars. We compare
the full sample of 5 candidates to theoretical expectations on the number
($\sim 10^8$) of BHs in the Milky Way and find reasonable agreement given the
small sample size.
