# Disclosing connections between black holes and naked singularities: Horizon remnants, Killing throats and bottlenecks

## TL;DR

This paper explores the relationships between black holes and naked singularities in Kerr spacetimes, introducing new concepts like Killing throats and bottlenecks to understand horizon structures and their formation.

## Contribution

It introduces the concepts of Killing throats and bottlenecks, and the extended plane, to analyze horizon remnants and the connection between black holes and naked singularities.

## Key findings

- Killing bottlenecks are linked to pre-horizon regimes.
- Metric bundles help reinterpret horizons across Kerr spacetimes.
- Frame-dragging effects influence horizon structures in extended planes.

## Abstract

We study the properties of black holes and naked singularities by considering stationary observers and light surfaces in Kerr spacetimes. We reconsider the notion of Killing horizons from a special perspective by exploring the entire family of Kerr metrics. To this end, we introduce the concepts of extended plane, Killing throats and bottlenecks for weak (slowly spinning) naked singularities. Killing bottlenecks (or horizon remnants in analogy with the corresponding definition of throats in black holes) are restrictions of the Killing throats appearing in special classes of slowly spinning naked singularities. Killing bottlenecks appear in association with the concept of pre-horizon regime introduced in [1, 2]. In the extended plane of the Kerr spacetime, we introduce particular sets, metric bundles, of metric tensors which allow us to reinterpret the concept of horizon and to find connections between black holes and naked singularities throughout the horizons. To evaluate the effects of frame-dragging on the formation and structure of Killing bottlenecks and horizons in the extended plane, we consider also the Kerr-Newman and the Reissner-Norstrom spacetimes. We argue that these results might be significant for the comprehension of processes that lead to the formation and eventually destruction of Killing horizons.