The OGLE-II event sc5₂859 -- An example of disk-disk microlensing

We present a new long-duration parallax event from the OGLE-II database, sc5_2859, which has the second longest time-scale ever identified (tE = 547.6{+22.6}{-7.8} days). We argue that both the lens and source reside in the Galactic disk, making event sc5_2859 one of the first confirmed examples of so-called disk-disk microlensing. We find that the source star is most probably located at a distance of D_S ~ 2 kpc, and from this we conclude that the lens is unlikely to be a main-sequence star due to the strict limits that can be placed on the lens brightness. A simple likelihood analysis is carried out on the lens mass, which indicates that the lens could be another candidate stellar mass black hole. We recommend that spectroscopic observations of the source be carried out in order to constrain the source distance, since this is the main source of uncertainty in our analysis. In addition, we briefly discuss whether there appears to be an excess of long duration microlensing events in the OGLE-II catalogue.