Dynamical Photon Spheres in Charged Black Holes and Naked Singularities

TL;DR

This paper develops an analytical model of dynamical photon spheres in charged black holes and naked singularities, revealing how charge affects photon sphere size and that naked singularities can cast shadows similar to black holes.

Contribution

It introduces a new analytical model for dynamical photon spheres in Bonnor-Vaidya spacetime and compares them with Vaidya spacetime, highlighting effects of charge and shadow casting by naked singularities.

Findings

Charge decreases photon sphere radius.

Naked singularities can cast shadows like black holes.

Shadow properties do not distinguish naked singularities from black holes.

Abstract

To understand the nature of a black hole shadow in dynamical spacetimes, we construct an analytical model of a dynamical photon sphere in the context of the Bonnor-Vaidya spacetime. Comparing the resulting photon sphere radius with the one in Vaidya spacetime, we find that the charge always decreases the radius of the photon sphere. We also prove that a naked singularity in Bonnor-Vaidya spacetime, unlike the static Reissner-Nordstrom naked singularity, may cast a shadow, and as a result, it cannot be distinguished from a black hole through its shadow.