A New Macho Search Strategy

I propose a radical revision in the search strategy for Massive Compact Objects (Machos) toward the Galactic bulge: monitor the entire $\sim 300$ square deg of the bulge and tune the search primarily to microlensing events of bright $M_V\text{lsim}2$ stars. By employing a small second telescope to follow up the events detected with low dimensionless impact parameter $\beta \text{lsim}0.2$, one could measure the proper motion $\mathrm{\Omega }$ of $\sim 10\mathrm{y}{\mathrm{r}}^{-1}$ events and significantly constrain $\mathrm{\Omega }$ for a similar number. If, in addition, 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. If a fraction $\xi$ of the bulge is composed of low-mass objects in the range ${10}^{-3}\text{lsim}{M}_{\mathrm{l}\mathrm{o}\mathrm{w}}\mathrm{/}{M}_{\odot }\text{lsim}{10}^{-1}$, then mass measurements could be made for $\sim 10(\xi/0.1)(0.01 M_\odot \VEV{M_{\rm low}^{-1}}),\rm yr^{-1}$ 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. The proposed strategy should therefore be adopted even before a parallax satellite is launched.