KMT-2016-BLG-2052L: Microlensing Binary Composed of M Dwarfs Revealed from a Very Long Time-scale Event

TL;DR

This paper analyzes a long-duration microlensing event revealing a binary system of M dwarfs, emphasizing the importance of combined measurements of parallax and Einstein radius to accurately determine lens masses.

Contribution

It demonstrates that long microlensing event durations can result from slow proper motion and large Einstein radii, not just heavy lenses, highlighting the need for combined measurements to identify massive remnants.

Findings

The lens is a binary of M dwarfs with masses ~0.34 and 0.17 solar masses.

Long event duration is due to slow proper motion and large Einstein radius, not heavy mass.

Heavy lenses (>1.0 M_sun) are a minor fraction (~19%) of long-duration events.

Abstract

We present the analysis of a binary microlensing event KMT-2016-BLG-2052, for which the lensing-induced brightening of the source star lasted for 2 seasons. We determine the lens mass from the combined measurements of the microlens parallax $\pie$ and angular Einstein radius $\thetae$. The measured mass indicates that the lens is a binary composed of M dwarfs with masses of $M_1\sim 0.34~M_\odot$ and $M_2\sim 0.17~M_\odot$. The measured relative lens-source proper motion of $\mu\sim 3.9~{\rm mas}~{\rm yr}^{-1}$ is smaller than $\sim 5~{\rm mas}~{\rm yr}^{-1}$ of typical Galactic lensing events, while the estimated angular Einstein radius of $\thetae\sim 1.2~{\rm mas}$ is substantially greater than the typical value of $\sim 0.5~{\rm mas}$. Therefore, it turns out that the long time scale of the event is caused by the combination of the slow $\mu$ and large $\thetae$ rather than the heavy mass of the lens. From the simulation of Galactic lensing events with very long time scales ($t_{\rm E}\gtrsim 100$ days), we find that the probabilities that long time-scale events are produced by lenses with masses $\geq 1.0~M_\odot$ and $\geq 3.0~M_\odot$ are $\sim 19\%$ and 2.6\%, respectively, indicating that events produced by heavy lenses comprise a minor fraction of long time-scale events. The results indicate that it is essential to determine lens masses by measuring both $\pie$ and $\thetae$ in order to firmly identify heavy stellar remnants such as neutron stars and black holes.