A NEW MACHO SEARCH STRATEGY

I propose a radical revision in the search strategy for massive compact objects (MACHOS) toward the Galactic bulge: monitor a large portion similar to 80 deg(2) of the bulge and tune the search primarily to microlensing events of bright M(I) less than or similar to 0.5 stars. There are three reasons for making this change. First, microlensing events of intrinsically bright sources have much smaller systematic uncertainties relative to the typical events now being monitored. Second, it is far more often possible to measure the proper motion of a MACHO when the source is intrinsically bright. By employing a small second telescope to follow up the events detected with low dimensionless impact parameter beta less than or similar to 0.2, one could measure the proper motion mu of similar to 20 yr(-1) events and significantly constrain mu for a similar number. Third, if the events were followed from a MACHO parallax satellite, it would be possible to measure individual masses, distances, and transverse velocities for the events with proper motions. Satellite parallaxes can in general be obtained only if the source is fairly bright. In addition, if a fraction xi of the bulge is composed of low-mass objects in the range 10(-3) less than or similar to M(low)/M. less than or similar to 10(-1). then mass measurements could be made for similar to 30(xi/0.1) (0.01 M. [M(low)(-1)]) yr(-1) of these, thus allowing a direct measurement of the mass function in the sub-stellar range. In the absence of a parallax satellite, ground-based observations could significantly constrain (but not measure) MACHO parallaxes. These constraints, when combined with the proper-motion measurements, would in turn constrain the mass, distance, and transverse speed of the MACHOS.