The ISCO of charged particles in Reissner-Nordstr\"om, Kerr-Newman and Kerr-Sen spacetime

TL;DR

This paper investigates the innermost stable circular orbit (ISCO) of charged particles around charged black holes in various spacetimes, revealing how charge interactions influence ISCO radius and stability limits.

Contribution

It provides a detailed analysis of how electric charge interactions affect the ISCO of particles in Reissner-Nordström, Kerr-Newman, and Kerr-Sen spacetimes, including effects of dilaton coupling.

Findings

ISCO radius increases with |qQ| for attractive interactions.

For repulsive interactions, ISCO radius first decreases then increases, diverging near qQ=1.

Stable bound orbits cease to exist for qQ ≥ 1 in Reissner-Nordström and Kerr-Newman spacetimes.

Abstract

In this article we study the innermost stable circular orbit (ISCO) of electrically charged particles in the electrically charged Reissner-Nordstr\"om spacetime, the Kerr-Newman spacetime and the Kerr-Sen spacetime. We find that the radius of the ISCO increases with an increasing particle-black hole charge product $|qQ|$ in the case of attractive Coulomb interaction $qQ<0$. For repulsive Coulomb interaction, the ISCO radius first decreases to a minimum and then increases again, until it diverges as the charge product approaches one. If the charge $Q$ of the black hole is very small, the minimum of the ISCO radius lies at $qQ=0$. Repulsive and attractive Coulomb interactions will always increase the ISCO radius in this limit. Stable bound orbits of charged particles cease to exist in the Reissner-Nordstr\"om and Kerr-Newman spacetime for $qQ\geq 1$. In the Kerr-Sen spacetime the limiting case depends on the charge of the black hole and if dilaton coupling is applied to the test particle. We find $qQ\geq 1+Q^2$ without dilaton-coupling and $qQ\geq 1+\frac{3}{2}Q^2$ with dilaton coupling $\alpha=1$.