Separability of the Hamilton-Jacobi equation and equatorial circular photon orbit in Kerr-Sen-Taub-NUT spacetime

TL;DR

This paper investigates the separability of the Hamilton-Jacobi equation and the properties of equatorial circular photon orbits in the Kerr-Sen-Taub-NUT spacetime, revealing a ring singularity outside the horizon.

Contribution

It demonstrates the additive separation of the Hamilton-Jacobi equation and explores photon orbits in the KSTN spacetime, highlighting the presence of a ring singularity.

Findings

Additive separation of Hamilton-Jacobi equation achieved.

Identified a ring singularity outside the horizon.

Analyzed equatorial circular photon orbits in KSTN spacetime.

Abstract

In this paper, we show the additive separation of the Hamilton-Jacobi equation, present the 4-velocity of the test particles, and attempt to find the equatorial circular photon orbit (ECPO) in the Kerr-Sen-Taub-NUT (KSTN) solution of the low energy limit of heterotic string theory. In the process of trying to find the ECPO, we instead encounter a ring singularity that could be outside the horizon of the KSTN black hole.