First Observation of Parallax in a Gravitational Microlensing Event

We present the first detection of parallax effects in a gravitational microlensing event. Parallax in a gravitational microlensing event observed only from the Earth appears as a distortion of the (otherwise symmetrical) light curve arising from the motion of the Earth around the Sun. This distortion can be detected if the event duration is not much less than a year and if the projected velocity of the lens is not much larger than the orbital velocity of the Earth about the Sun. The event presented here has a duration (or Einstein diameter crossing time) of t̂ = 220 days and clearly shows the distortion resulting from the Earth's motion. We find that the projected velocity of the lens is = 75 ± 5 km s-1 at an angle of 28° ± 4° from the direction of increasing galactic longitude, as expected for a lens in the galactic disk. A likelihood analysis of this event yields estimates of the distance to and mass of the lens: Dlens = 1.7+1.1−0.7 kpc and M = 1.3+1.3−0.6 M☉. This suggests that the lens is a remnant such as a white dwarf or neutron star. It is possible, though less likely, that the lens is a main-sequence star. If so, we can add our upper limit on the observed flux from the lens to the analysis. This modifies the estimates to Dlens = 2.8+1.1−0.6 kpc and M = 0.6+0.4−0.2 M☉.