Theory of Exploring the Dark Halo with Microlensing. I. Power-Law Models

If microlensing of stars by dark matter has been detected, then the way is open for the development of new methods in galactic astronomy. This series of papers investigates what microlensing can teach us about the structure and shape of the dark halo. In this paper we present formulae for the microlensing rate, optical depth, and event duration distributions for a simple set of axisymmetric disk-halo models. The halos are based on the "power-law models" of Evans which have simple velocity distributions. Using these models, we show that there is a large uncertainty in the predicted microlensing rate because of uncertainty in the halo parameters. For example, models which reproduce the measured galactic observables to within their errors still differ in microlensing rate toward the Magellanic Clouds by more than a factor of 10. We find that while the more easily computed optical depth correlates well with microlensing rate, the ratio of optical depth to rate can vary by a factor of 2 (or greater if the disk is maximal). Comparison of microlensing rates toward the Large and Small Magellanic Clouds (LMC and SMC) and M31 can be used to aid determinations of the halo flattening and rotation curve slope. For example, the ratio of microlensing rates toward the LMC and SMC is ∼0.7-0.8 for E0 halos and ∼1.0-1.2 for E7 halos. Once the flattening has been established, the ratio of microlensing rates toward M3 1 and the LMC may help to distinguish between models with rising, flat, or falling rotation curves. Comparison of rates along LMC and galactic bulge lines of sight gives useful information on the halo core radius, although this may not be so easy to extract in practice. Maximal disk models provide substantially smaller halo optical depths, shorter event durations, and even larger model uncertainties.