Binary microlensing event OGLE-2009-BLG-020 gives a verifiable mass, distance and orbit predictions

TL;DR

This paper demonstrates a method to verify binary microlensing measurements using Doppler observations, enabling precise determination of lens properties and testing microlensing models.

Contribution

It introduces a framework combining microlensing and Doppler data to accurately measure binary lens parameters and test microlensing solutions.

Findings

First verifiable binary microlensing event with Doppler data

Measurement of binary mass, orbit, and microlens parallax

Overconstrained system allowing strong model verification

Abstract

We present the first example of binary microlensing for which the parameter measurements can be verified (or contradicted) by future Doppler observations. This test is made possible by a confluence of two relatively unusual circumstances. First, the binary lens is bright enough (I=15.6) to permit Doppler measurements. Second, we measure not only the usual 7 binary-lens parameters, but also the 'microlens parallax' (which yields the binary mass) and two components of the instantaneous orbital velocity. Thus we measure, effectively, 6 'Kepler+1' parameters (two instantaneous positions, two instantaneous velocities, the binary total mass, and the mass ratio). Since Doppler observations of the brighter binary component determine 5 Kepler parameters (period, velocity amplitude, eccentricity, phase, and position of periapsis), while the same spectroscopy yields the mass of the primary, the combined Doppler + microlensing observations would be overconstrained by 6 + (5 + 1) - (7 + 1) = 4 degrees of freedom. This makes possible an extremely strong test of the microlensing solution. We also introduce a uniform microlensing notation for single and binary lenses, we define conventions, summarize all known microlensing degeneracies and extend a set of parameters to describe full Keplerian motion of the binary lenses.