OGLE-2017-BLG-0329L: A Microlensing Binary Characterized with Dramatically Enhanced Precision Using Data from Space-based Observations

Mass measurements of gravitational microlensesrequire one to determine the microlens parallaxπ_E, but precise π_E measurement, in many cases, is hampered due to the subtlety of the microlens- parallaxsignal combined with the difficulty of distinguishing the signal from those induced by other higher-order effects. In this work, we present the analysis of the binary-lens event OGLE-2017-BLG-0329, for which π_E is measured with a dramatically improved precision using additional data from space-based Spitzer observations. We find that while the parallaxmodel based on the ground-based data cannot be distinguished from a zero-π E model at the 2σ level, the addition of the Spitzer data enables us to identify two classes of solutions, each composed of a pair of solutions according to the well-known eclipticdegeneracy. It is found that the space-based data reduce the measurement uncertainties of the north and east components of the microlens- parallaxvector π_E by factors ~18 and ~4, respectively. With the measured microlens parallaxcombined with the angular Einstein radiusmeasured from the resolved caustic crossings, we find that the lens is composed of a binary with component masses of either (M_1, M_2) ~ (1.1, 0.8) M⊙ or ~(0.4, 0.3) M⊙ according to the two solution classes. The first solution is significantly favored but the second cannot be securely ruled out based on the microlensing data alone. However, the degeneracy can be resolved from adaptive optics observations taken ~10 years after the event.