Null orbits and shadows in the Ernst-Wild geometry: insights for black holes immersed in a magnetic field

TL;DR

This paper explores how magnetic fields and rotation influence the light rings and shadows of Kerr-Newman black holes, providing both numerical and analytical insights into their observable features and stability properties.

Contribution

It introduces a combined numerical and analytical study of shadows in magnetized Kerr-Newman black holes, revealing the effects of magnetic fields and rotation on light rings and shadows.

Findings

Magnetic fields and rotation significantly alter black hole shadows.

Analytical perturbation matches numerical results for shadow deviations.

Connections established between light ring stability and quasinormal modes.

Abstract

We investigate the null geodesics, in particular the stable and unstable light rings and shadows, of a Kerr-Newman black hole immersed in an asymptotically uniform magnetic field as described by the Ernst-Wild (Melvin-Kerr-Newman) spacetime. Through numerical ray tracing, we demonstrate that both the black hole rotation and the magnetized Melvin geometry impact the light rings and shadows non-trivially and in compensating ways. In addition, we use a perturbative expansion in the magnetic field B to analyze the deviation of the observable shadow relative to the Kerr result analytically, and determine connections between Lyapunov exponents for light ring instabilities and quasinormal modes in the eikonal limit.