Using Orbital Effects to Break the Close/wide Degeneracy in Binary-lens Microlensing Events

TL;DR

This paper introduces a method to resolve the close/wide degeneracy in binary-lens microlensing events by analyzing the orbital effects on light curves, demonstrated on a real event to identify the lens composition.

Contribution

The paper presents a novel approach using orbital effects to distinguish between close and wide binary microlenses, overcoming a longstanding degeneracy problem.

Findings

Successfully applied the method to a real event, resolving the degeneracy.

Determined the lens system consists of K and M-type dwarfs at ~3.5 kpc.

Provided a unique characterization of the binary lens system.

Abstract

Microlensing can provide an important tool to study binaries, especially those composed of faint or dark objects. However, accurate analysis of binary-lens light curves is often hampered by the well-known degeneracy between close (s<1) and wide (s>1) binaries, which can be very severe due to an intrinsic symmetry in the lens equation. Here s is the normalized projected binary separation. In this paper, we propose a method that can resolve the close/wide degeneracy using the effect of a lens orbital motion on lensing light curves. The method is based on the fact that the orbital effect tends to be important for close binaries while it is negligible for wide binaries. We demonstrate the usefulness of the method by applying it to an actually observed binary-lens event MOA-2011-BLG-040/OGLE-2011-BLG-0001, which suffers from severe close/wide degeneracy. From this, we are able to uniquely specify that the lens is composed of K and M-type dwarfs located at ~3.5 kpc from the Earth.