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 \pi _ { E } and angular Einstein radius \theta _ { E } .
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 ~ { } { mas } ~ { } { yr } ^ { -1 } is smaller than \sim 5 ~ { } { mas } ~ { } { yr } ^ { -1 } of typical Galactic lensing events , while the estimated angular Einstein radius of \theta _ { E } \sim 1.2 ~ { } { mas } is substantially greater than the typical value of \sim 0.5 ~ { } { mas } .
Therefore , it turns out that the long time scale of the event is caused by the combination of the slow \mu and large \theta _ { E } rather than the heavy mass of the lens .
From the simulation of Galactic lensing events with very long time scales ( t _ { 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 \pi _ { E } and \theta _ { E } in order to firmly identify heavy stellar remnants such as neutron stars and black holes .