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Free keywords:
binaries: general
gravitational lensing: micro
Astrophysics - Solar
and Stellar Astrophysics
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 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.
