A POSSIBLE BINARY SYSTEM OF A STELLAR REMNANT IN THE HIGH-MAGNIFICATION GRAVITATIONAL MICROLENSING EVENT OGLE-2007-BLG-514

We report the extremely high-magnification (A > 1000) binary microlensing event OGLE-2007-BLG-514. We obtained good coverage around the double peak structure in the light curve via follow-up observations from different observatories. The binary lens model that includes the effects of parallax (known orbital motion of the Earth) and orbital motion of the lens yields a binary lens mass ratio of <i>q</i> = 0.321 0.007 and a projected separation of <i>s</i> = 0.072 0.001 in units of the Einstein radius. The parallax parameters allow us to determine the lens distance <i>D<sub>L</sub></i> = 3.11 0.39kpc and total mass <i>M<sub>L</sub></i> = 1.40 0.18 <i>M</i>⊙; this leads to the primary and secondary components having masses of <i>M</i><sub>1</sub> = 1.06 0.13 <i>M</i>⊙ and <i>M</i><sub>2</sub> = 0.34 0.04 <i>M</i>⊙, respectively. The parallax model indicates that the binary lens system is likely constructed by the main-sequence stars. On the other hand, we used a Bayesian analysis to estimate probability distributions by the model that includes the effects of xallarap (possible orbital motion of the source around a companion) and parallax (<i>q</i> = 0.270 0.005, <i>s</i> = 0.083 0.001). The primary component of the binary lens is relatively massive, with <i>M</i><sub>1</sub> = 0.9<span style="position:relative;bottom:0.6em;font-size:0.8em;">+4.6</span><span style="position:relative;top:0.3em;font-size:0.8em;margin-left:-1.8em;">-0.3</span> <i>M</i>⊙ and it is at a distance of <i>D<sub>L</sub></i> = 2.6<span style="position:relative;bottom:0.6em;font-size:0.8em;">+3.8</span><span style="position:relative;top:0.3em;font-size:0.8em;margin-left:-1.8em;">-0.9</span> kpc. Given the secure mass ratio measurement, the companion mass is therefore <i>M</i><sub>2</sub> = 0.2<span style="position:relative;bottom:0.6em;font-size:0.8em;">+1.2</span><span style="position:relative;top:0.3em;font-size:0.8em;margin-left:-1.8em;">-0.1</span> <i>M</i>⊙. The xallarap model implies that the primary lens is likely a stellar remnant, such as a white dwarf, a neutron star, or a black hole.