MACHO Mass Determination Based on Space Telescope Observation

We investigate the possibility of lens mass determination for a causticcrossing microlensing event based on a space telescope observation. Wedemonstrate that the parallax due to the orbital motion of a space telescopecauses a periodic fluctuation of the light curve, from which the lens distancecan be derived. Since the proper motion of the lens relative to the source isalso measurable for a caustic crossing event, one can find a full solution formicrolensing properties of the event, including the lens mass. To determine thelens mass with sufficient accuracy, the light curve near the caustic crossingshould be observed within uncertainty of $\sim$ 1%. We argue that the HubbleSpace Telescope observation of the caustic crossing supplied with ground-basedobservations of the full light curve will enable us to determine the mass ofMACHOs, which is crucial for understanding the nature of MACHOs.Comment: 9 pages + 3 figures, accepted for publication in ApJ Letter