OGLE-2017-BLG-1038: A Possible Brown-dwarf Binary Revealed by Spitzer Microlensing Parallax

TL;DR

This paper analyzes a microlensing event revealing a possible brown-dwarf binary, with space-based data enabling physical parameter estimation despite degeneracies, and suggests future observations to resolve these uncertainties.

Contribution

It introduces a detailed analysis of a microlensing event with degeneracies in mass estimates and proposes methods to resolve them using future infrared adaptive optics imaging.

Findings

Identification of very-low-mass binary solutions near the brown dwarf boundary.

Degeneracy between low-mass brown dwarf binary and higher-mass M-dwarf binary solutions.

Future infrared imaging can resolve the degeneracy within ten years.

Abstract

We report the analysis of microlensing event OGLE-2017-BLG-1038, observed by the Optical Gravitational Lensing Experiment, Korean Microlensing Telescope Network, and Spitzer telescopes. The event is caused by a giant source star in the Galactic Bulge passing over a large resonant binary lens caustic. The availability of space-based data allows the full set of physical parameters to be calculated. However, there exists an eightfold degeneracy in the parallax measurement. The four best solutions correspond to very-low-mass binaries near ($M_1 = 170^{+40}_{-50} M_J$ and $M_2 = 110^{+20}_{-30} M_J$), or well below ($M_1 = 22.5^{+0.7}_{-0.4} M_J$ and $M_2 = 13.3^{+0.4}_{-0.3} M_J$) the boundary between stars and brown dwarfs. A conventional analysis, with scaled uncertainties for Spitzer data, implies a very-low-mass brown dwarf binary lens at a distance of 2 kpc. Compensating for systematic Spitzer errors using a Gaussian process model suggests that a higher mass M-dwarf binary at 6 kpc is equally likely. A Bayesian comparison based on a galactic model favors the larger-mass solutions. We demonstrate how this degeneracy can be resolved within the next ten years through infrared adaptive-optics imaging with a 40 m class telescope.