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 time sampling of 1-2 times a week this is sufficient for early detection of caustic crossings. Nearly real time photometry of QSO 2237+0305 is available from the OGLE web site http://www.astro.princeton.edu/~ogle/ogle2/huchra.html . During the 1999 observing season, the apparent V magnitude of the A, B, C and D images changed by 0.50, 0.15, 0.65 and 0.35 mag, respectively. Most likely however, none of the microlensing events involved a caustic crossing. The most rapid variation was 0.25 mag in 30 days, observed for image C. The alert system will continue to be active in the next observing season from late April until September 2000, when OGLE suspends operation for an upgrade. Observations will resume for season of 2001.