Observers in Kerr spacetimes: the ergoregion on the equatorial plane

TL;DR

This paper investigates stationary observers in the ergosphere of Kerr spacetimes, revealing key differences between black holes and naked singularities, and deriving limiting frequencies and properties of zero angular momentum observers.

Contribution

It provides a detailed analysis of stationary observers in Kerr ergospheres, distinguishing between black holes and naked singularities, and introduces new limiting angular frequencies for naked singularities.

Findings

Differences between black hole and super-spinner sources in ergospheres

Limiting angular frequencies for naked singularities

Properties of zero angular momentum observers

Abstract

We perform a detailed analysis of the properties of stationary observers located on the equatorial plane of the ergosphere in a Kerr spacetime, including light-surfaces. This study highlights crucial differences between black hole and the super-spinner sources. In the case of Kerr naked singularities, the results allow us to distinguish between "weak" and "strong" singularities, corresponding to spin values close to or distant from the limiting case of extreme black holes, respectively. We derive important limiting angular frequencies for naked singularities. We especially study very weak singularities as resulting from the spin variation of black holes. We also explore the main properties of zero angular momentum observers for different classes of black hole and naked singularity spacetimes.