Secondary caustics in close multiple lenses

TL;DR

This paper analyzes the complex caustic structures in close multiple lens systems, deriving formulas for secondary caustics and exploring their geometries using perturbative methods.

Contribution

It introduces a general perturbative approach to characterize secondary caustics in close multiple lens systems, including arbitrary configurations and the binary case.

Findings

Derived formulas for the number, position, and shape of secondary caustics.

Identified unique geometries in specific lens configurations.

Simplified expressions for the binary lens case.

Abstract

We investigate the caustic structure of a lens composed by a discrete number of point-masses, having mutual distances smaller than the Einstein radius of the total mass of the system. Along with the main critical curve, it is known that the lens map is characterized by secondary critical curves producing small caustics far from the lens system. By exploiting perturbative methods, we derive the number, the position, the shape, the cusps and the area of these caustics for an arbitrary number of close multiple lenses. Very interesting geometries are created in some particular cases. Finally we review the binary lens case where our formulae assume a simple form.