KMT-2016-BLG-2052L: Microlensing Binary Composed of M Dwarfs Revealed from a Very Long Timescale Event
© 2018. The American Astronomical Society. All rights reserved. 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, πE, and angular Einstein radius, θE. The measured mass indicates that the lens is a binary composed of M dwarfs with masses of M1∼ 0.34 Moand M2∼ 0.17 Mo. The measured relative lens-source proper motion of μ ∼ 3.9 mas yr-1is smaller than ∼5 mas yr-1of typical Galactic lensing events, while the estimated angular Einstein radius of θE∼ 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 θErather than the heavy mass of the lens. From the simulation of Galactic lensing events with very long timescales (tE≳ 100 days), we find that the probabilities that long timescale events are produced by lenses with masses ≥1.0 Moand ≥3.0 Moare ∼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 πEand θEin order to firmly identify heavy stellar remnants, such as neutron stars and black holes.