Test of the string loop oscillation model using kHz quasiperiodic oscillations in a neutron star binary

TL;DR

This paper tests the string loop oscillation model against high-frequency QPOs in a neutron star binary, showing it can explain observed frequencies within certain neutron star parameter ranges.

Contribution

It demonstrates that the string loop oscillation model can account for observed kHz QPOs in a neutron star binary, with specific constraints on neutron star parameters and string properties.

Findings

Model explains observed frequencies for 0.2<a<0.4 and 2.1<M/M_sun<2.5

The parameter ω varies for different QPO observations

External Kerr geometry approximation introduces less than 10% error

Abstract

The model of current-carrying string loop oscillations is tested to explain the special set of frequencies related to the high-frequency quasiperiodic oscillations (HF QPOs) observed recently in the low-mass X-ray binary XTE J1701-407 containing a neutron star. The external geometry of the neutron star is approximated by the Kerr geometry, introducing errors not exceeding $10~\%$ for slowly rotating massive neutron stars. The frequencies of the radial and vertical string loop oscillations are then governed by the mass $M$ and dimensionless spin $a$ of the neutron star, and by the dimensionless parameter $\omega$ describing combined effects of the string loop tension and its angular momentum. It is explicitly demonstrated that the string-loop oscillation model can explain the observed kHz frequencies for the neutron star parameters restricted to the intervals ${0.2<a<0.4}$ and ${2.1<M/{\rm M}_{\odot}<2.5}$. However, the stringy parameter $\omega$ cannot be the same for all the three HF QPO observations in the XTE J1701-407 source; the limits on the acceptable values of $\omega$ are given in dependence on the spacetime parameters $M$ and $a$.