OGLE-2017-BLG-0039: Microlensing Event with Light from a Lens Identified from Mass Measurement

We present an analysis of the caustic-crossing binary microlensing event OGLE-2017-BLG-0039. Thanks to the very long duration of the event, with a time scale t <SUB>E</SUB> ˜ 130 days, the microlens parallax is measured precisely despite its low value of {π }<SUB>{{E</SUB>}}˜ 0.06. Analysis of the well-resolved caustic crossings during the source star’s entrance and exit of the caustic yields an angular Einstein radius of {θ }<SUB>{{E</SUB>}}˜ 0.6 mas. The measured {π }<SUB>{{E</SUB>}} and {θ }<SUB>{{E</SUB>}} indicate that the lens is a binary composed of two stars with masses ˜ 1.0 {M}<SUB>⊙ </SUB> and ˜0.15 M <SUB>⊙</SUB>, and is located at a distance of ˜6 kpc. From the color and brightness of the lens estimated from its determined mass and distance, it is expected that ˜2/3 of the I-band blended flux comes from the lens. Therefore, the event is a rare case of a bright lens event for which high-resolution follow-up observations can confirm its nature.