Visualizing event horizon and ergosphere of Kerr black hole

Importing required modules

import numpy as np
import astropy.units as u
import matplotlib.pyplot as plt
from einsteinpy.utils import kerr_utils, schwarzschild_radius

Defining the black hole charecteristics

M = 4e30
scr = schwarzschild_radius(M * u.kg).value
# for nearly maximally rotating black hole
a1 = 0.499999*scr
# for ordinary black hole
a2 = 0.3*scr

Calculating the ergosphere and event horizon for spherical coordinates

ergo1, ergo2, hori1, hori2 = list(), list(), list(), list()
thetas = np.linspace(0, np.pi, 720)
for t in thetas:
    ergo1.append(kerr_utils.radius_ergosphere(M, a1, t, "Spherical"))
    ergo2.append(kerr_utils.radius_ergosphere(M, a2, t, "Spherical"))
    hori1.append(kerr_utils.event_horizon(M, a1, t, "Spherical"))
    hori2.append(kerr_utils.event_horizon(M, a2, t, "Spherical"))
ergo1, ergo2, hori1, hori2 = np.array(ergo1), np.array(ergo2), np.array(hori1), np.array(hori2)

Calculating the X, Y coordinates for plotting

Xe1, Ye1 = ergo1[:,0] * np.sin(ergo1[:,1]), ergo1[:,0] * np.cos(ergo1[:,1])
Xh1, Yh1 = hori1[:,0] * np.sin(hori1[:,1]), hori1[:,0] * np.cos(hori1[:,1])
Xe2, Ye2 = ergo2[:,0] * np.sin(ergo2[:,1]), ergo2[:,0] * np.cos(ergo2[:,1])
Xh2, Yh2 = hori2[:,0] * np.sin(hori2[:,1]), hori2[:,0] * np.cos(hori2[:,1])

Plot for maximally rotating black hole

%matplotlib inline
fig, ax = plt.subplots()
# for maximally rotating black hole
ax.fill(Xh1, Yh1, 'b', Xe1, Ye1, 'r', alpha=0.3)
ax.fill(-1*Xh1, Yh1, 'b', -1*Xe1, Ye1, 'r', alpha=0.3)

[<matplotlib.patches.Polygon at 0x7fd4307cfed0>,
 <matplotlib.patches.Polygon at 0x7fd4307db150>]

Plot for rotating(normally) black hole

%matplotlib inline
fig, ax = plt.subplots()
ax.fill(Xh2, Yh2, 'b', Xe2, Ye2, 'r', alpha=0.3)
ax.fill(-1*Xh2, Yh2, 'b', -1*Xe2, Ye2, 'r', alpha=0.3)

[<matplotlib.patches.Polygon at 0x7fd430817d50>,
 <matplotlib.patches.Polygon at 0x7fd4308dfe50>]

• The inner body represents event horizon and outer one represents ergosphere. It can be concluded that with decrease in angular momentum, radius of event horizon increases, and that of ergosphere decreases.