A Natural Formalism for Microlensing

Abstract

If the standard microlensing geometry is inverted so that the Einstein ring is projected onto the observer plane rather than the source plane, then the relations between the observables (θ_E,r_E) and the underlying physical quantities (M,πrel) become immediately obvious. Here θ_E and r_E are the angular and projected Einstein radii, M is the mass of the lens, and π_rel is the lens-source relative parallax. I recast the basic formalism of microlensing in light of this more natural geometry and in terms of observables. I then find that the relations between observable and physical quantities assume an exceptionally simple form. In an appendix, I propose a set of notational conventions.