# Null Geodesics and Observables around Kerr-Sen Black Hole

## TL;DR

This paper analyzes the geodesic motion and observable phenomena like light bending and perihelion precession around Kerr-Sen Black Holes, revealing how black hole parameters influence photon orbits and related astrophysical signals.

## Contribution

It provides a detailed study of photon and test particle orbits around Kerr-Sen Black Holes, including non-equatorial motion and observable effects, extending previous analyses to this specific black hole solution.

## Key findings

- No terminating photon orbits due to black hole charge.
- Derived conditions for various photon orbits based on black hole parameters.
- Calculated black hole mass and rotation from photon red/blue shifts.

## Abstract

We investigate the geodesic motion in the background of Kerr-Sen Black Hole arising in the heterotic string theory. The nature of effective potential is discussed in radial as well as latitudinal direction. A special class of spherical photon orbits is obtained along with the expression for the turning point for radial photons. Dependence of photon motion within this class of solution is discussed explicitly in view of the different Black Hole parameters. We have discussed the allowed regions for geodesic motion of massless test particles around Kerr- Sen Black Hole in more generalised way by including non-equatorial motion of the photons into the account. The conditions for different types of possible orbits are discussed with specific parameter values along with the corresponding orbit structure. No terminating orbits are possible for photons due to non-zero Black Hole charge. Observables on the angular plane (viz. bending of light and perihelion precession for massive test particles) are analysed as special cases. We have also calculated the rotation and mass parameters for Kerr-Sen Black Hole in terms of the red/blue shifts of the photons in circular and equatorial orbits emitted by the massive test particles which represent stars or other probable sources of photons.