Quasi-Equatorial Gravitational Lensing by Spinning Black Holes in the Strong Field Limit

TL;DR

This paper investigates how the spin of black holes affects strong field gravitational lensing, revealing complex caustic structures and the potential dominance of a single relativistic image for high spins.

Contribution

It derives a two-dimensional lens equation for spinning black holes near the equatorial plane and analyzes the resulting caustic structures and observational implications.

Findings

Black hole spin causes caustics to drift and extend.

High spin can lead to observing only one relativistic image.

Non equatorial images may significantly influence lensing phenomenology.

Abstract

Spherically symmetric black holes produce, by strong field lensing, two infinite series of relativistic images, formed by light rays winding around the black hole at distances comparable to the gravitational radius. In this paper, we address the relevance of the black hole spin for the strong field lensing phenomenology, focusing on trajectories close to the equatorial plane for simplicity. In this approximation, we derive a two-dimensional lens equation and formulae for the position and the magnification of the relativistic images in the strong field limit. The most outstanding effect is the generation of a non trivial caustic structure. Caustics drift away from the optical axis and acquire finite extension. For a high enough black hole spin, depending on the source extension, we can practically observe only one image rather than two infinite series of relativistic images. In this regime, additional non equatorial images may play an important role in the phenomenology.