Photon emission of extremal Kerr-Newman black holes

TL;DR

This paper analytically studies the null geodesics near the horizon of extremal Kerr-Newman black holes, focusing on photon trajectories and their observable shifts, using matched asymptotic expansions.

Contribution

It provides an analytical solution for null geodesics near the superradiant bound in extremal Kerr-Newman black holes, especially for equatorial photon trajectories.

Findings

Analytical expressions for azimuthal and time shifts of photons

Solutions valid near the superradiant bound

Insights into photon paths from near-horizon to distant observers

Abstract

In this paper, we deal with the null geodesics extending from the near-horizon region out to a distant observatory in an extremal Kerr-Newman black hole background. In particular, using the matched asymptotic expansion method, we analytically solve the null geodesics near the superradiant bound in the form of algebraic equations. For the case that the photon trajectories are limited in the equatorial plane, the shifts in the azimuthal angle and time are obtained.