High magnification events by MOA in 2007

Abstract

Gravitational microlensing events of high magnification provide exceptional sensitivity to the presence of low-mass planets orbiting the lens star, including planets with masses as low as that of Earth. The essential requirement for the detection of such planets in these events is that the FWHM of the light curve be monitored continuously, or as nearly continuously as possible. The dependence of planet detectability on the magnification caused by microlensing, on the planet mass and planet location, and on the size of the source star, may be understood in terms of simple geometrical properties of microlensing that have been known since 1964. Planetary signals of low-mass planets are found to be approximately independent of the magnification caused by microlensing. This implies that planets can be detected in events over a wide range of magnifications, from moderately high values ~ 100 to very high values ~ 1000. The former values are likely to yield more clear-cut separations of the stellar and planetary features on the light curve, but they require larger telescopes to obtain precision photometry. During 2007, twenty-four events with magnification exceeding 50 were detected by the MOA collaboration, of which about half were also detected by the OGLE collaboration. A quarter of the events received essentially continuous coverage of their FWHMs by follow-up collaborations, another quarter received partial coverage, and the remaining half received little coverage. Final analysis of these events is still underway, but casual inspection of the light curves reveals some possible planetary detections amongst them. During 2008 it is hoped that fuller coverage of events of high magnification will be obtained with the addition of further telescopes to existing follow-up networks.