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

TL;DR

This paper investigates how a Gaussian-distributed plasma in the polar direction affects the shadow of Kerr black holes, revealing that plasma density and distribution significantly influence shadow size and shape.

Contribution

It introduces a novel model of plasma distribution around Kerr black holes and analyzes its impact on black hole shadows using numerical ray-tracing methods.

Findings

Shadow size varies with observer inclination angle.

Radial plasma density distribution affects shadow shape.

Plasma acts as a convex lens altering the shadow.

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 $1$.