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

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.