Potential Direct Single-Star Mass Measurement

We analyze the light curve of the microlensing event OGLE-2003-BLG-175/MOA- 2003-BLG-45 and show that it has two properties that, when combined with future high-resolution astrometry, could lead to a direct, accurate measurement of the lens mass. First, the light curve shows clear signs of distortion due to the Earth's accelerated motion, which yields a measurement of the projected Einstein radius r̃ E. Second, from precise astrometric measurements, we show that the blended light in the event is coincident with the microlensed source to within about 15 mas. This argues strongly that this blended light is the lens and hence opens the possibility of directly measuring the lens-source relative proper motion μ rel and so the mass M = (c 2/4G)μ relt Er̃ E, where t E is the measured Einstein timescale. While the light-curve-based measurement of r̃ E is, by itself, severely degenerate, we show that this degeneracy can be completely resolved by measuring the direction of proper motion μ rel.