Investigating the geometry of quasars with microlensing

When a source crosses the caustics of a gravitational microlens at cosmological distances, a high-magnification event occurs. This is seen as a change in the total flux of the image of the source. We present an analytic result for the magnification of a point source near a parabolic fold caustic. This is a higher-order approximation to the existing solution for a straight fold caustic. We show from a study of fold-crossing events that we can distinguish between disc and annular source geometries. One important application of this work will be the determination of the geometric structure of the luminous regions of quasars. By observing the flux changes of a quasar image during a microlensing event, it is possible to determine features of quasar structure that would otherwise be below the resolution limit available with existing optical methods.