Shadows of Kerr black holes with a Gaussian-distributed plasma in the polar direction

Abstract

In this work, we study the shadow of Kerr black hole surrounded by an axisymmetric plasma, whose density takes a Gaussian distribution in the polar direction. Along the radial direction, we consider two models: in model A the density of the plasma decays in a power law; in model B the density obeys a logarithmic Gaussian distribution. Using the numerical backward ray-tracing method, we find that the size of the shadow is sensitive to the inclination angle of the observer due to the polar distribution of the density of the plasma. In particular, we pay special attention to the model B and investigate the influence of the radial position of maximum density, the decay rate of the density towards the event horizon and the opening angle of the plasma on the shape and size of the Kerr black hole shadow. The effects of the plasmas studied in this work can be qualitatively explained by taking the plasmas as convex lenses with the refractive index being less than $1$.