Halo structure, masses of dark objects and parallax microlensing

Abstract

We study the use of parallax microlensing to separate the effects of the mass function of dark massive halo objects (MHOs or `machos&#39;) on the one hand and their spatial distribution and kinematics on the other. This disentanglement is supposed to allow a much better determination of the two than could be achieved entirely on the basis of the durations of events. We restrict our treatment to the same class of power-law spherical models for the halo of MHOs studied in a previous paper (astro-ph/9609187). Whereas the duration-based error in the average MHO mass, μexceeds (at N=100 events) μby a factor of 2 or more, parallax microlensing remarkably brings it down to 15-20% of μ, regardless of the shape of the mass function. In addition, the slope αof the mass function, dn/dμ\proptoμ^α, can be inferred relatively accurately (σ_α < 0.4) for a broader range, -3 <α< 0. The improvement in the inference of the halo structure is also significant: the index γof the density profile (ρ\sim R^{-γ}) can be obtained with the error σ_γ <0.4. While in a typical situation the errors for the parameters specifying the velocity dispersion profile are of about the same magnitude as the parameters themselves, virtually all the uncertainty is `concentrated&#39; in linear combinations of the parameters that may have little influence on the profile and thus allow its reasonably accurate inference.