Ground-based parallax confirmed by Spitzer: binary microlensing event MOA-2015-BLG-020

TL;DR

This paper confirms ground-based parallax measurements of a binary microlensing event using Spitzer satellite data, enabling precise determination of the lens's physical parameters and demonstrating the effectiveness of combined observational methods.

Contribution

It provides the first confirmation of ground-based parallax measurements with satellite observations in a binary microlensing event, improving lens parameter accuracy.

Findings

Binary lens composed of two dwarf stars with known masses

Lens located at approximately 2.44 kpc in the Galactic disk

Ground and space-based parallax measurements are consistent

Abstract

We present the analysis of the binary gravitational microlensing event MOA-2015-BLG-020. The event has a fairly long timescale (about 63 days) and thus the light curve deviates significantly from the lensing model that is based on the rectilinear lens-source relative motion. This enables us to measure the microlensing parallax through the annual parallax effect. The microlensing parallax parameters constrained by the ground-based data are confirmed by the Spitzer observations through the satellite parallax method. By additionally measuring the angular Einstein radius from the analysis of the resolved caustic crossing, the physical parameters of the lens are determined. It is found that the binary lens is composed of two dwarf stars with masses $M_1 = 0.606 \pm 0.028M_\odot$ and $M_2 = 0.125 \pm 0.006M_\odot$ in the Galactic disk. Assuming the source star is at the same distance as the bulge red clump stars, we find the lens is at a distance $D_L = 2.44 \pm 0.10 kpc$. In the end, we provide a summary and short discussion of all published microlensing events in which the annual parallax effect is confirmed by other independent observations.