OGLE-2017-BLG-0537: Microlensing Event with a Resolvable Lens in $\lesssim 5$ years from High-resolution Follow-up Observations

TL;DR

This paper analyzes a binary-lens microlensing event with a high proper motion, estimating lens properties and demonstrating the potential to resolve the lens from the source within five years through follow-up observations.

Contribution

The study provides detailed analysis of a high proper motion microlensing event and predicts the feasibility of directly imaging the lens in about five years.

Findings

Lens components are separated by ~1.3 times the angular Einstein radius.

Proper motion is the third highest among measured microlensing events.

Estimated lens masses are approximately 0.4 and 0.2 solar masses, at 1.2 kpc distance.

Abstract

We present the analysis of the binary-lens microlensing event OGLE-2017-BLG-0537. The light curve of the event exhibits two strong caustic-crossing spikes among which the second caustic crossing was resolved by high-cadence surveys. It is found that the lens components with a mass ratio $\sim 0.5$ are separated in projection by $\sim 1.3\thetae$, where $\thetae$ is the angular Einstein radius. Analysis of the caustic-crossing part yields $\thetae=1.77\pm 0.16$~mas and a lens-source relative proper motion of $\mu =12.4\pm 1.1~{\rm mas}~{\rm yr}^{-1}$. The measured $\mu$ is the third highest value among the events with measured proper motions and $\sim 3$ times higher than the value of typical Galactic bulge events, making the event a strong candidate for follow-up observations to directly image the lens by separating it from the source. From the angular Einstein radius combined with the microlens parallax, it is estimated that the lens is composed of two main-sequence stars with masses $M_1\sim 0.4~M_\odot$ and $M_2\sim 0.2~M_\odot$ located at a distance of $D_{\rm L}\sim 1.2$~kpc. However, the physical lens parameters are not very secure due to the weak microlens-parallax signal, and thus we cross check the parameters by conducting a Bayesian analysis based on the measured Einstein radius and event timescale combined with the blending constraint. From this, we find that the physical parameters estimated from the Bayesian analysis are consistent with those based on the measured microlens parallax. Resolving the lens from the source can be done in about 5 years from high-resolution follow-up observations and this will provide a rare opportunity to test and refine the microlensing model.