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

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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.
Funding Information
  • NSERC, Canada

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