The First Direct Detection of a Gravitational μ-Lens toward the Galactic Bulge

We present a direct detection of the gravitational lens that caused the microlensing event MACHO-95-BLG-37. This is the first fully resolved microlensing system involving a source in the Galactic bulge, and the second such system in general. The lens and source are clearly resolved in images taken with the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST) ~9 yr after the microlensing event. The currently available data are not sufficient for the final, unambiguous identification of the gravitational lens and the microlensed source. While the light-curve models combined with the high-resolution photometry for individual objects indicate that the source is red and the lens is blue, the color-magnitude diagram for the line of sight and the observed proper motions strongly support the opposite case. The first scenario points to a metal-poor lens with mass M ≈ 0.6 M☉ at the distance Dl ≈ 4 kpc. In the second scenario, the lens could be a main-sequence star with M = 0.8-0.9 M☉ about halfway to the Galactic bulge or in the foreground disk, depending on the extinction.