Multiband photometry of microlensing event Kojima-1

We measure the microlens parallax vector {pi}_E_ using Spitzer and ground-based light curves with constraints on the direction of lens-source relative proper motion derived from Very Large Telescope Interferometer (VLTI) GRAVITY observations. Combining this {pi}_E_ determination with the angular Einstein radius {theta}_E_ measured by VLTI-GRAVITY observations, we find that the lens is a star with mass M_L_=0.495{+/-}0.063M{odot} at a distance DL=429{+/-}21pc.

We find that the blended light basically all comes from the lens. The lens-source proper motion is {mu}_rel,hel_=26.55{+/-}0.36mas/yr, so with currently available adaptive-optics instruments, the lens and source can be resolved in 2021. This is the first microlensing event whose lens mass is unambiguously measured by interferometry+satellite-parallax observations, which opens a new window for mass measurements of isolated objects such as stellar-mass black holes.