First Resolution of Microlensed Images

We employ Very Large Telescope Interferometer GRAVITY to resolve, for the first time, the two images generated by a gravitational microlens. The measurements of the image separation Delta theta(-,+) = 3.78 +/- 0.05 mas, and hence the Einstein radius theta(E) = 1.87 +/- 0.03 mas, are precise. This demonstrates the robustness of the method, provided that the source is bright enough for GRAVITY (K less than or similar to 10.5) and the image separation is of order of or larger than the fringe spacing. When theta(E) is combined with a measurement of the "microlens parallax" pi(E), the two will together yield the lens mass and lens-source relative parallax and proper motion. Because the source parallax and proper motion are well measured by Gaia, this means that the lens characteristics will be fully determined, whether or not it proves to be luminous. This method can be a powerful probe of dark, isolated objects, which are otherwise quite difficult to identify, much less characterize. Our measurement contradicts Einstein's prediction that "the luminous circle [i.e., microlensed image] cannot be distinguished" from a star.