MACHO MASSES FROM SATELLITE-OBSERVATIONS

Microlensing events in which a distant star is lensed by a dark object in the disc, bulge, or halo of our Galaxy can potentially reveal thre e important properties of the lensing object, namely its mass, velocit y, and distance from us. Detections of such events in the past have be en able to determine these properties only by appealing to statistical properties of the events and comparing these with a model of the halo objects. Improvements in astrometry have led others to propose method s of breaking the degeneracy of these parameters. Whilst these methods are ingenious, we describe a simpler technique achieving the same res ults. The observations required would be the motion of the centroid of the lensed star (i.e., the centre of brightness of the lensed image), previously described by Walker(1), and the times of peak amplificatio n as seen by two observers, with a baseline separation of order 10(8) km (for example an Earth-based observer and a satellite in a Jupiter-l ike orbit). In this paper we show how the observations from this satel lite can be used to reveal the lens properties.