Killing horizons, throats and bottlenecks in the ergoregion of the Kerr spacetime

Abstract: The properties of Kerr black holes (BHs)and naked singularities (NSs) are investigated by using stationary observers and their limiting frequencies. We introduce the concept of NS Killing throats and bottlenecks for slowly spinning NSs to describe the frequency of stationary observers. In particular, we show the frequency on the horizon can be used to point out a connection between BHs and NSs and to interpret the horizon in terms of frequencies. The analysis is performed on the equatorial plane of the ergoregion.