Test particle motion around black hole in Einstein-Maxwell-scalar theory

TL;DR

This paper investigates test particle motion around black holes in Einstein-Maxwell-scalar theory, analyzing spacetime structure, photon spheres, ISCO radii, and particle frequencies, revealing potential observational constraints on the theory.

Contribution

It provides a detailed analysis of particle orbits and stability in EMS black holes, highlighting differences from Kerr black holes and potential observational tests.

Findings

ISCO radius for neutral particles decreases with black hole charge

Charged particle ISCO radius increases with coupling parameter, conflicting with observations

Kerr black hole spin can mimic EMS black hole properties up to a/M ≈ 0.936

Abstract

In this paper, we explore the test particle motion around a black hole in Einstein-Maxwell-scalar (EMS) theory using three different black hole solutions within this theory. We have first analyzed the spacetime curvature structure of these solutions and shown the existence of two singularities and the first one is at the center $r=0$. In black hole spacetime, there are two regions divided by the critical value of the cosmological parameter $\lambda_0$. The photon sphere around the black hole in EMS theory has also been studied and found that it does not depend on cosmological parameter $\lambda$. We have analyzed the innermost stable circular orbits (ISCO) around the black hole and shown that for all solutions ISCO radius for neutral particle decreases with the increase of black hole charge. We have also studied the charged particle motion around the black hole where charged particle motion is considered in the presence of the gravitational field and the Coulomb potential. It is shown that ISCO radius for charged particles increases depending on the selected value of the coupling parameter which is in contradiction with observations of the inner edge of the accretion disks of the astrophysical black holes and can be used as a powerful tool to rule out the EMS theory from consideration for the gravitational field theory. It is also studied the fundamental frequencies governed by test particle orbiting around the black hole in EMS theory. Finally, as a test of black hole solution in EMS theory ISCO radii is compared with that in Kerr black hole and found that the spin parameter of Kerr can be mimic up to $a/M\simeq 0.936$.