Discovering Galactic planets by gravitational microlensing: magnification patterns and light curves

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Abstract
The current searches for microlensing events towards the Galactic bulge can be used to detect planetary companions around the main lensing stars. The effect of such planets is a short-term modulation on the smooth microlensing light curve produced by the main lensing star. Current and planned experiments should be sensitive enough to be able to discover planets with masses ranging from the mass of Jupiter down to the mass of the Earth. In order to be able to successfully detect planets in this way, it is necessary to accurately and frequently monitor a microlensing event photometrically, once it has been ‘triggered’. Here we present a large variety of two-dimensional magnification distributions for systems consisting of an ordinary star and a planetary companion. We cover planet/ star mass ratios from mpl/M⋆ = 10−5 to 10−3. These limits correspond roughly to MEarth and MJupiterM, for a typical lens mass of M⋆ ≈0.3 M⊙. We explore a range of star-planet distances, with particular emphasis on the case of ‘resonant lensing’, a situation in which the planet is located at, or very near, the Einstein ring of the lensing star. We show a wide selection of light curves — one-dimensional cuts through the magnification patterns — to illustrate the broad range of possible light curve perturbations caused by planets. The strongest effects are to be expected for caustic crossings. However, even tracks passing outside the caustics can have a considerable effect on the light curves. The easiest detectable (projected) distance range for the planets is between approximately 0.6 and 1.6 Einstein radii. Planets in this distance range produce caustics inside the Einstein ring of the star. For a typical lens (source) distance of about 6 kpc (8 kpc), this corresponds to a physical distance between star and planet of approximately 1 to 3 au, for a lensing star with a mass of about 0.3 M⊙.
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