Multiple Schwarzschild lensing represents a very rich terrain for the exploration of caustics in gravitational lensing. The occurrence of different kinds of singularities stimulates new investigations.
In this paper we have applied perturbative methods to secondary caustics, forming when the masses are close each other with respect to the total Einstein radius. In this way we have been able to establish the number of the caustics for any lens configuration, the positions and the shapes, with a complete characterization of the geometries arising in all cases. Moreover, quantitative formulae for the area and other features of these objects have been given. As we have seen, in the most common case, the shape of the simple caustics is always triangular. Anyway, multiple caustics exist developing a great variety of behaviours, giving rise, to curves having a number of cusps ranging from four to infinity. The breaking of multiple caustics can follow different ways depending on how the parameters of the system change.
© European Southern Observatory (ESO) 2000
Online publication: June 30, 2000