When the Stars Align — the First Resolved Microlensed Images

Abstract

Using GRAVITY, we have resolved the two images of a microlensed source star for the first time, more than a century after Einstein first predicted that such image splitting could be caused by the gravity of another (lens) star along the line of sight to the source. We have measured the angular Einstein radius (almost exactly half the image separation) to be 1.87 milliarcseconds, with a precision of just 30 microarcseconds. The measurement also yields the direction of the relative motion of the lens with respect to the source. These results, combined with other, so-called microlens parallax measurements, yield the lens mass and distance. While this lens is an ordinary luminous star, the same technique could be applied in the future to measure the mass and distance of completely dark objects, such a black hole. In fact, while black holes in binaries have been found from X-ray and LIGO gravitational-wave observations, and are likely to be found in the future by Gaia astrometry, gravitational microlensing is the only known way to find isolated black holes. Our detection using GRAVITY on the VLTI opens the path to such measurements of isolated black hole masses.