Interpretation of the OGLE Q2237+0305 microlensing light curve (1997-1999)

The four bright images of the gravitationally lensed quasar Q2237+0305 are being monitored from the ground (e.g. OGLE collaboration, Apache Point Observatory) in the hope of observing a high-magnification event (HME). Over the past three seasons (1997-1999) the OGLE collaboration has produced microlensing light curves with unprecedented coverage. These demonstrate smooth, independent (therefore microlensing) variability between the images. We have retrospectively applied probability functions for HME parameters to the observed light-curve features. We conclude that the 1999 image C peak was as a result of the source having passed outside of a cusp rather than to a caustic crossing. In addition, we find that the image C light curve shows evidence for a caustic crossing between the 1997 and the 1998 observing seasons involving the appearance of new critical images. Our models predict that the next image C event is most likely to arrive 500d following the 1999 peak, but with a large uncertainty (~100-2000d). Finally, given the image A light-curve derivative at the end of the 1999 observing season, our modelling suggests that a caustic crossing will occur between the 1999 and the 2000 observing seasons, suggesting a minimum for the image A light curve ~1-1.5mag fainter than the 1999 November level.