KMT-2016-BLG-2052L: Microlensing Binary Composed of M Dwarfs Revealed from a Very Long Timescale 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 two seasons. We determine the lens mass from the combined measurements of the microlens parallax, π _E, and angular Einstein radius, θ _E. The measured mass indicates that the lens is a binary composed of M dwarfs with masses of M ₁ ∼ 0.34 M _⊙ and M ₂ ∼ 0.17 M _⊙. The measured relative lens-source proper motion of μ ∼ 3.9 mas yr^-1 is smaller than ∼5 mas yr^-1 of typical Galactic lensing events, while the estimated angular Einstein radius of θ _E ∼ 1.2 mas is substantially greater than the typical value of ∼0.5 mas. Therefore, it turns out that the long timescale of the event is caused by the combination of the slow μ and large θ _E rather than the heavy mass of the lens. From the simulation of Galactic lensing events with very long timescales (t _E ≳ 100 days), we find that the probabilities that long timescale events are produced by lenses with masses ≥1.0 M _⊙ and ≥3.0 M _⊙ are ∼19% and 2.6%, respectively, indicating that events produced by heavy lenses comprise a minor fraction of long timescale events. The results indicate that it is essential to determine lens masses by measuring both π _E and θ _E in order to firmly identify heavy stellar remnants, such as neutron stars and black holes.