Limits of Binaries That Can Be Characterized by Gravitational Microlensing

TL;DR

This paper investigates the limitations of gravitational microlensing in characterizing binary systems, revealing that parameter degeneracies hinder the detection and analysis of wide-separation binaries beyond certain limits.

Contribution

It identifies the separation thresholds where binary lensing parameters can be reliably characterized, highlighting the degeneracy issues in high-magnification microlensing events.

Findings

Binarity detectable up to ~60 Einstein radii for equal-mass binaries.

Parameter determination feasible only for binaries within ~20 Einstein radii.

Degeneracy complicates characterization of wide-separation binaries.

Abstract

Due to the high efficiency of planet detections, current microlensing planet searches focus on high-magnification events. High-magnification events are sensitive to remote binary companions as well and thus a sample of wide-separation binaries are expected to be collected as a byproduct. In this paper, we show that characterizing binaries for a portion of this sample will be difficult due to the degeneracy of the binary-lensing parameters. This degeneracy arises because the perturbation induced by the binary companion is well approximated by the Chang-Refsdal lensing for binaries with separations greater than a certain limit. For binaries composed of equal mass lenses, we find that the lens binarity can be noticed up to the separations of $\sim 60$ times of the Einstein radius corresponding to the mass of each lens. Among these binaries, however, we find that the lensing parameters can be determined only for a portion of binaries with separations less than $\sim 20$ times of the Einstein radius.