Ogle - 2000 - Bul - 43 : a Spectacular Ongoing Parallax Microlensing Event . Difference Image Analysis

We present the photometry and theoretical models for a Galactic bulge microlensing event OGLE-2000-BUL-43. The event is very bright with I = 13.54 mag, and has a very long time scale, t E = 156 days. The long time scale and its light curve deviation from the standard shape strongly suggest that it may be affected by the parallax effect. We show that OGLE-2000-BUL-43 is the first discovered microlensing event, in which the parallax distortion is observed over a period of 2 years. Difference Image Analysis (DIA) using the PSF matching algorithm of Alard & Lupton enabled photometry accurate to 0.5%. All photometry obtained with DIA is available electronically. Our analysis indicates that the viewing condition from a location near Jupiter will be optimal and can lead to magnifications ∼ 50 around January 31, 2001. These features offer a great promise for resolving the source (a K giant) and breaking the degeneracy between the lens parameters including the mass of the lens, if the event is observed with the imaging camera on the Cassini space probe. Gravitational microlensing was originally proposed as a method of detecting compact dark matter objects in the Galactic halo (Paczy´nski 1986). However, it also turned out to be an extremely useful method to study Galactic structure, mass functions of stars and potentially extra-solar planetary systems (for a review, see Paczy´nski 1996). Most microlensing events are well described by the standard light curve (e.g., Paczy´nski 1986). Unfortunately, from these light curves, one can derive only a single physical constraint, namely the Einstein radius crossing time, which involves the lens mass, various distance measures and relative velocity (see §4). This degeneracy means that the lens properties cannot be uniquely inferred. Therefore any further information on the lens configuration is of great importance. Microlensing events that exhibit paral-lax effects provide this type of information. Such effects can occur when the event is observed simultaneously from two different positions in the Solar system (Refsdal 1966) or when the event lasts long enough that the Earth's motion can no longer be approximated as rectilinear during the event (Gould 1992). Both of these effects will be directly relevant to the current paper. The first parallax microlensing event was reported by the MACHO collaboration toward the Galactic bulge (Alcock et al. 1995), and the second case (toward Carina) was discovered by the OGLE collaboration and reported in Mao (1999). Additional parallax microlensing …