Microlens Parallaxes with SIRTF

The Space Infrared Telescope Facility (SIRTF) will drift away from the Earth at ∼ 0.1 AU yr. Microlensing events will therefore have different characteristics as seen from the satellite and the Earth. From the difference, it is possible in principle to measure ṽ, the transverse velocity of the lens projected onto the observer plane. Since ṽ has very different values for different populations (disk, halo, Large Magellanic Cloud), such measurements could help identify the location, and hence the nature, of the lenses. I show that the method previously developed by Gould for measuring such satellite parallaxes fails completely in the case of SIRTF: it is overwhelmed by degeneracies which arise from fact that the Earth and satellite observations are in different band passes. I develop a new method which allows for observations in different band passes and yet removes all degeneracies. The method combines a purely ground-based measurement of the “parallax asymmetry” with a measurement of the delay between the time the event peaks at the Earth and satellite. In effect, the parallax asymmetry determines the component of ṽ in the Earth-Sun direction, while the delay time measures the component of ṽ in the direction of the Earth’s orbit. Subject headings: dark matter – Galaxy: halo – gravitational lensing – Magellanic Clouds