First Resolution of Microlensed Images*

Abstract

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 mas, and hence the Einstein radius θ 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 ≲ 10.5) and the image separation is of order of or larger than the fringe spacing. When θ E  is combined with a measurement of the “microlens parallax” , 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.