Binary events and extragalactic planets in pixel microlensing

Abstract

Pixel microlensing, i.e. gravitational microlensing of unresolved stars, can be used to explore distant stellar systems, and as a bonus may be able to detect extragalactic planets. In these studies, binary-lens events with multiple high-magnification peaks are crucial. Considering only those events which exhibit caustic crossings, we estimate the fraction of binary events in several example pixel microlensing surveys and compare them to the fraction of binary events in a classical survey with resolved stars. We find a considerable enhancement of the relative rate of binary events in pixel microlensing surveys, relative to surveys with resolved sources. We calculate the rate distribution of binary events with respect to the time between caustic crossings. We consider possible surveys of M31 with ground-based telescopes and of M87 with HST and NGST. For the latter, a pixel microlensing survey taking one image a day may observe of order one dozen binary events per month.