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Massive Compact Halo Objects (MACHOs) might comprise dark galactic halos, including that of our Galaxy. When a MACHO passes in front of a background star, it reveals itself through gravitational lensing, which results in an amplification of the star's light producing a unique, color independent light curve. Recent surveys using stars in the LMC and in the bulge of the Galaxy have indicated several candidate MACHO events.
We suggest a new strategy for the observations: when a ground based observatory picks up a light curve indicative of a MACHO event, the Hubble Space Telescope (HST) triggers into action and records a complementary light curve. A comparison of two light curves could show a difference due to the orbital velocity (8 km/sec) difference. This additional velocity, which corresponds to about 5 percent of the typical MACHO velocity, could help better determine the true velocity of the lensing object and may help facilitate the determination of the true mass of the MACHO.
The presence of a second, moving telescope is most discriminating when the duration of the event is small due to a small (or fast) lens, e.g. takes place all in one night. The light curve must be sampled many times during the event. A different observer velocity will lead to different relative positions over a sequence of time frames in the lensing event, which can result in different measured light curves for each observer. In this case, there is additional information in the two light curves which may yield a better estimate of relative lens velocity. However, the photometry would have to be good enough to see this effect. The relative difference in magnitudes may be large enough for large magnification events.
The longer the baseline between the space-based and terrestrial observers, the larger the difference in the light curves will be. Thus, the velocity information would be more easily discerned from the light curves measured by missions both further from the earth's surface and moving the fastest relative to the ground based observatory.
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