Orbital and epicyclic frequencies around rapidly rotating compact stars in scalar-tensor theories of gravity

TL;DR

This paper investigates how scalar-tensor theories of gravity alter the orbital and epicyclic frequencies around rapidly rotating neutron and strange stars, highlighting potential tests of strong gravity through astrophysical observations.

Contribution

It introduces the first models of rapidly rotating, scalarized compact stars with realistic equations of state and derives analytical expressions for their exterior spacetime and frequencies.

Findings

Large deviations in frequencies for maximum-mass models compared to GR.

Small deviations at 700Hz rotation for typical models.

Potential for using frequency deviations to test strong gravity theories.

Abstract

We study the orbital and epicyclic frequencies of particles orbiting around rapidly rotating neutron stars and strange stars in a particular scalar-tensor theory of gravity. We find very large deviations of these frequencies, when compared to their corresponding values in general relativity, for the maximum-mass rotating models. In contrast, for models rotating with spin frequency of 700Hz (approximately the largest known rotation rate of neutron stars), the deviations are generally small. Nevertheless, for a very stiff equation of state and a high mass the deviation of one of the epicyclic frequencies from its GR value is appreciable even at a spin frequency of 700Hz. In principle, such a deviation could become important in models of quasi-periodic oscillations in low-mass x-ray binaries and could serve as a test of strong gravity (if other parameters are well constraint). Even though the present paper is concentrated mainly on orbital and epicyclic frequencies, we present here for the first time rapidly rotating, scalarized equilibrium compact stars with realistic hadronic equations of state and strange matter equation of state. We also provide analytical expressions for the exterior spacetime of scalarized neutron stars and their epicyclic frequencies in the nonrotating limit.