Direct detection of a microlens in the Milky Way

The nature of dark matter remains mysterious, with luminous material accoun ting for at most similar to 25 per cent of the baryons in the Universe(1,2) . We accordingly undertook a survey looking for the microlensing of stars i n the Large Magellanic Cloud (LMC) to determine the fraction of Galactic da rk matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sig ht. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance a nd transverse velocity of the lens. The survey results to date indicate tha t between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected fr equency of nearby stars acting as lenses, and demonstrates a direct determi nation of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MA CHOs.