# Innermost stable circular orbits of charged spinning test particles

## TL;DR

This paper investigates how spin and electric charge influence the innermost stable circular orbits of particles in Kerr-Newman spacetime, revealing complex relationships and potential degeneracy of orbits for particles with identical spin but different charges.

## Contribution

It provides a detailed analysis of the combined effects of spin and charge on particle orbits, highlighting non-monotonous behaviors and orbit degeneracy.

## Key findings

- Angular momentum and energy depend monotonically on spin and charge.
- Particle radius can change non-monotonously with charge.
- Particles with same spin but different charges can share the same last stable orbit.

## Abstract

The effects of a paritcle's spin and electric charge on its angular momentum, energy and radius on the innermost stable circular orbit are investigated based on the particle's equations of motion in a background of the Kerr-Newmann spacetime. It is found that the particle's angular momentum and energy have monotonous relationships with not only its spin but also its charge; it is also discovered that the spinning particle's radius may change non-monotonously with its charge. Hence, our result remarkably indicates that particles owning identical spin but different charge may degenerate into a same last stable circular orbit.