Rate of Binary Events in Microlensing

The consideration of binary lenses is crucial in the analysis of self-lensing in microlensing surveys. A binary event presents high magnification peaks when the source crosses the caustic curve. We compute the rates of caustic crossing events both for resolved sources, such as towards the galactic bulge, and for unresolved sources, as in pixel microlensing. We find that the relative number of caustic crossing events in a pixel microlensing survey is comparable to that in a classical survey with resolved stars. The two competing effects of larger region of high magnification and more severe finite source effects approximately cancel out. A pixel microlensing survey of M87 with the Advanced Camera for Surveys to be installed on the Hubble Space Telescope could observe several caustic crossing events in a month of daily monitoring, and a comparable NGST survey could observe of order 50 such events. Furthermore, such a search might be able to detect extragalactic planetary systems. Subject headings: binaries:general — gravitational lensing