Constraining models of twin peak quasi-periodic oscillations with realistic neutron star equations of state

TL;DR

This study tests various neutron star equations of state against twin-peak QPO models, using detailed spacetime calculations to constrain neutron star properties and exclude incompatible models based on observational data.

Contribution

It extends previous work by incorporating realistic neutron star equations of state and detailed spacetime effects to better constrain QPO models and neutron star parameters.

Findings

Certain EoS are incompatible with observed QPOs at specific spin frequencies.

Approximately 13 EoS are compatible with twin-peak QPOs and the relativistic precession model.

Only 5 EoS remain compatible when considering low frequency QPOs and Lense-Thirring precession.

Abstract

Twin-peak quasi-periodic oscillations (QPOs) are observed in the X-ray power-density spectra of several accreting low-mass neutron star (NS) binaries. In our previous work we have considered several QPO models. We have identified and explored mass-angular-momentum relations implied by individual QPO models for the atoll source 4U 1636-53. In this paper we extend our study and confront QPO models with various NS equations of state (EoS). We start with simplified calculations assuming Kerr background geometry and then present results of detailed calculations considering the influence of NS quadrupole moment (related to rotationally induced NS oblateness) assuming Hartle-Thorne spacetimes. We show that the application of concrete EoS together with a particular QPO model yields a specific mass-angular-momentum relation. However, we demonstrate that the degeneracy in mass and angular momentum can be removed when the NS spin frequency inferred from the X-ray burst observations is considered. We inspect a large set of EoS and discuss their compatibility with the considered QPO models. We conclude that when the NS spin frequency in 4U 1636-53 is close to 580Hz we can exclude 51 from 90 of the considered combinations of EoS and QPO models. We also discuss additional restrictions that may exclude even more combinations. Namely, there are 13 EOS compatible with the observed twin peak QPOs and the relativistic precession model. However, when considering the low frequency QPOs and Lense-Thirring precession, only 5 EOS are compatible with the model.