Black hole shadow in an asymptotically-flat, stationary, and axisymmetric spacetime: The Kerr-Newman and rotating regular black holes

TL;DR

This paper presents a formula for calculating black hole shadows in stationary, axisymmetric spacetimes and demonstrates its application to Kerr-Newman and rotating regular black holes, aiding observational identification.

Contribution

It introduces a versatile formula for black hole shadow contours applicable to various stationary, axisymmetric black holes, including new rotating black hole examples.

Findings

The formula accurately reproduces known black hole shadow contours.

Application to Kerr-Newman black holes confirms its validity.

New shadow contours for rotating regular black holes are obtained.

Abstract

The shadow of a black hole can be one of the strong observational evidences for stationary black holes. If we see shadows at the center of galaxies, we would say whether the observed compact objects are black holes. In this paper, we consider a formula for the contour of a shadow in an asymptotically-flat, stationary, and axisymmetric black hole spacetime. We show that the formula is useful for obtaining the contour of the shadow of several black holes such as the Kerr-Newman black hole and rotating regular black holes. Using the formula, we can obtain new examples of the contour of the shadow of rotating black holes if assumptions are satisfied.