The rate of caustic crossing microlensing events for Q2237+0305

Spectrophotometric observations of the gravitationally microlensed quasar Q2237+0305 during a High Magnification Event (HME) is potentially a very powerful tool for probing the structure of the quasars accretion disc on scales of ∼ 10 arc seconds. In cases where the HME is produced by a single caustic (SHME), microlensing induced changes in the spectrum during the event may be used to directly infer the source intensity profile (eg. Grieger et al. 1988; Yonehara et al. 1988; Agol & Krolik 1999a). Several groups are actively monitoring Q2237+0305 with this goal in mind. How often we can expect to observe a HME is dependent on the lens system parameters of galactic transverse velocity, mean microlens mass and the size of the magnified continuum source. We have previously used published microlensed lightcurves to obtain expressions for the likely-hood of the values for these parameters (Wyithe, Webster & Turner 1999b,c; Wyithe, Webster, Turner & Mortlock 1999). Here we use this information to investigate the expected rate of SHMEs. We find the average rate of SHMEs as well as the number that we can expect to observe over periods of a decade and of a single observing season. We find that the average SHME rate summed over all images in Q2237+0305 is 1.5±0.6 6.2±1.3 events per decade. During the period following a caustic crossing we find that the event rate in the corresponding image is enhanced by ∼ 50− 100%, and therefore that the overall event rate may be higher during these periods. From the distribution of events expected during a 6 month period we find that there is 1 chance in ∼ 4− 10 of observing a SHME per observing season. The systematic dependence in these values arises from the different assumptions for smooth matter content, orientation of the galactic transverse velocity and the size of photometric error in the monitoring data. The results support continued monitoring of Q2237+0305 with the aim of obtaining detailed spectroscopic and photometric observations of a SHME.