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

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 $2.1<M/{\mathrm{M}}_{\odot }<2.5$. 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 $a$.

24 pages, 6 figures

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