Gravitational Lensing by Black Holes with Multiple Photon Spheres

TL;DR

This paper investigates gravitational lensing effects caused by hairy black holes with two photon spheres, revealing complex image structures and critical curves that enhance understanding of black hole optics.

Contribution

It presents the first detailed analysis of lensing by hairy black holes with multiple photon spheres, highlighting new image formations and critical curve features.

Findings

Two photon spheres can produce multiple high-order images.

Two critical curves appear in the black hole image, with the shadow edge on the smaller one.

Additional higher-order images are observed inside and outside the larger critical curve.

Abstract

We study gravitational lensing of light by hairy black holes, which, in a certain parameter regime, can possess two photon spheres of different size outside the event horizon. In particular, we focus on higher-order images of a point-like light source and a luminous celestial sphere produced by strong gravitational lensing near photon spheres. Two photon spheres usually triple the number of high-order images of a point-like light source. When a hairy black hole is illuminated by a celestial sphere, two photon spheres would give rise to two critical curves in the black hole image, and the smaller critical curve coincides with the shadow edge. In addition to a set of higher-order images of the celestial sphere outside the shadow edge, two more sets of higher-order images are observed inside and outside the larger critical curve, respectively.