Iterative solutions for the gravitational lens equation in the strong deflection limit

Abstract

Two exact lens equations have been recently shown to be equivalent to each other, being consistent with the gravitational deflection angle of light from a source to an observer, both of which can be within a finite distance from a lens object [Phys. Rev. D 102, 064060 (2020)]. We examine methods for iterative solutions of the gravitational lens equations in the strong deflection limit. It has been so far unclear whether a convergent series expansion can be provided by the gravitational lens approach based on the geometrical optics for obtaining approximate solutions in the strong deflection limit in terms of a small offset angle. By using the ratio of the lens mass to the lens distance, we discuss a slightly different method for iterative solutions and behavior of the convergence. Finite-distance effects begin at the third order in the iterative method. The iterative solutions in the strong deflection limit are estimated for Sgr ${\mathrm{A}}^{*}$ and M87. These results suggest that only the linear-order solution can be relevant with current observations, while the finite-distance effects at the third order may be negligible in the Schwarzschild lens model for these astronomical objects.