MOA-2011-BLG-293Lb: A testbed for pure survey microlensing planet detections

Microlensing uses gravitational lensing of individual stars to detect planets. In a microlensing event, the light from one star, the “source”, is magnified by the gravitational field of another star, the “lens”. A microlensing event occurs when the paths of two stars on the sky pass very close to each other, typically less than 1′′ [An arcsecond (′′) is 1/3600th of a degree. It may be further divided into mas=10−3′′ or μas=10−6′′.]. The magnification of the source star depends on its projected distance from the lens star, with the magnification increasing for smaller projected distances. The right-hand panel of Figure 1 shows the magnification map for a solitary lens star (a point lens), with the lens star at the center. The grayscale corresponds to the magnification of a source at that position with darker grays indicating higher magnification. Because the stars are moving, we see the brightness of the source change as a function of time because its magnification changes with its projected distance from the lens as it traces a path across this map. This “light curve” reflects the gravitational field of the lens. If there is a planet orbiting the lens star, it can create distortions in the light curve, allowing the planet to be detected. The left-hand panel of Figure 1 shows a magnification map for a planet+star lens. The difference between this map and the map for a point lens shows the effect of the planet; far from the center, the two maps are nearly identical.