The optical gravitational lensing experiment: A hunt for caustic crossings in QSO 2237+0305

In 1998, the Optical Gravitational Lensing Experiment (OGLE) successfully implemented automated data reductions for QSO 2237+0305. Using a new image-subtraction method, we achieved a differential photometry scatter of 1%-5% for images A-D, respectively. Combined with a sampling rate of one to two points a week, this is sufficient for early detection of caustic crossings. A nearly real-time photometry of QSO 2237+0305 is available from the OGLE Web site.(2) During the 1999 observing season, the difference between the highest and the lowest apparent V magnitude of the A, B, C, and D images was 0.50, 0.15, 0.65, and 0.35 mag, respectively. However, the most likely interpretation is that none of the recorded microlensing events were a bona fide caustic crossing. The most rapid variation was a 0.25 mag decrease in 30 days, which was observed for image C after it peaked in early July of 1999. The alert system will continue to be active in the current observing season from late April until September of 2000, when OGLE suspends operation for an upgrade. Observations will resume for the season of 2001.