Shadows and lensing of black holes immersed in strong magnetic fields

TL;DR

This paper explores how strong magnetic fields influence black hole shadows, light rings, and photon orbits, revealing unique phenomena like panoramic shadows and chaotic lensing in magnetized black hole spacetimes.

Contribution

It provides a detailed analysis of null geodesics, light rings, and shadows of black holes in strong magnetic fields, including the novel finding of panoramic shadows and the behavior of photon orbits.

Findings

Weak magnetic fields cause oblate shadows and prolate horizons.

Strong magnetic fields eliminate LRs outside the horizon, leading to panoramic shadows.

Chaotic lensing and multiple disconnected shadows are observed.

Abstract

We investigate the null geodesic flow and in particular the light rings (LRs), fundamental photon orbits (FPOs) and shadows of a black hole (BH) immersed in a strong, uniform magnetic field, described by the Schwarzschilld-Melvin electrovacuum solution. The empty Melvin magnetic Universe contains a tube of planar LRs. Including a BH, for weak magnetic fields, the shadow becomes oblate, whereas the intrinsic horizon geometry becomes prolate. For strong magnetic fields (over-critical solutions), there are no LRs outside the BH horizon, a result explained using topological arguments. This feature, together with the light confining structure of the Melvin Universe yields \textit{panoramic shadows}, seen (almost) all around the equator of the observer's sky. Despite the lack of LRs, there are FPOs, including polar planar ones, which define the shadow edge. We also observe and discuss chaotic lensing, including in the empty Melvin Universe, and multiple disconnected shadows.