SATELLITE PARALLAXES OF LENSING EVENTS TOWARD THE GALACTIC BULGE

In order to understand the nature of the lenses that generate microlen sing events, one would Like to measure their mass, distance, and veloc ity. Unfortunately, current microlensing experiments measure only one parameter of the events, the characteristic timescale, which is a comb ination of the underlying physical parameters. Other methods are requi red to extract additional information. Parallax measurements using a s atellite in an Earth-like orbit yield the projected velocity of the le ns: <(nu)over tilde> = nu/(1 - z), where nu is the transverse velocity (speed and direction) of the lens relative to the Earth-source line o f sight, and z is the ratio of the distances to the lens and the sourc e. A measurement of <(nu)over tilde> could distinguish between lenses belonging to the bulge and disk populations. We show that for photomet ric precisions of 1%-2%, it is possible to measure the projected speed , <(nu)over tilde>, to an accuracy of less than or equal to 10% for ov er 70% of disk lenses and over 60% of bulge lenses. For measuring the projected velocity <(nu)over tilde>, the percentages are 40% and 30%, respectively. We find lines of sight greater than 2 degrees away from the ecliptic are preferable, and an Earth-satellite separation in the range 0.7 AU-1.9 AU is optimal. The requirements of the satellite for measuring the projected velocities of events toward the bulge are simi lar to those for measurements toward the LMC.