Optical Gravitational Lensing Experiment. OGLE-1999-BUL-19: The First Multi-Peak Parallax Event

We describe a highly unusual microlensing event, OGLE-1999-BUL-19. Unlike most standard microlensing events, this event exhibits multiple peaks in its light curve. The Einstein radius crossing time for this event is approximately one year, which is unusually long. We show that the additional peaks in the light curve can be caused by the very small value for the relative transverse velocity of the lens projected into the observer plane (ṽ ≈ 12.5± 1.1 km s). Since this value is significantly less than the speed of the Earth’s orbit around the Sun (v⊕ ∼ 30 km s), the motion of the Earth induces these multiple peaks in the light curve. This value for ṽ is the lowest velocity so far published and we believe that this is the first multiple-peak parallax event ever observed. We also found that the event can be somewhat better fitted by a rotating binary-source model, although this is to be expected since every parallax microlensing event can be exactly reproduced by a suitable binarysource model. A face-on rotating binary-lens model was also identified, but this provides a significantly worse fit. We conclude that the most-likely cause for this multi-peak behaviour is due to parallax microlensing rather than microlensing by a binary source. However, this event may be exhibiting slight binary-source signatures in addition to these parallax-induced multiple peaks. With spectroscopic observations it is possible to test this ‘parallax plus binary-