We report the detection and mass measurement of a binary lens OGLE-2015-BLG-1285La , b , with the more massive component having M _ { 1 } > 1.35 M _ { \odot } ( 80 % probability ) .
A main-sequence star in this mass range is ruled out by limits on blue light , meaning that a primary in this mass range must be a neutron star or black hole .
The system has a projected separation r _ { \perp } = 6.1 \pm 0.4 { AU } and lies in the Galactic bulge .
These measurements are based on the “ microlens parallax ” effect , i.e. , comparing the microlensing light curve as seen from Spitzer , which lay at 1.25 { AU } projected from Earth , to the light curves from four ground-based surveys , three in the optical and one in the near infrared .
Future adaptive optics imaging of the companion by 30m class telescopes will yield a much more accurate measurement of the primary mass .
This discovery both opens the path and defines the challenges to detecting and characterizing black holes and neutron stars in wide binaries , with either dark or luminous companions .
In particular , we discuss lessons that can be applied to future Spitzer and Kepler K2 microlensing parallax observations .