HF QPOs in the neutron star binary system XTE J1701-462 fitted by the model of oscillating string loops

Axisymmetric string loops oscillating around a stable equilibrium position in the Kerr background are applied to explain the special set of frequencies related to the high-frequency quasiperiodic oscillations observed in the low mass X-ray binary XTE J1701-462 containing a neutron star. Frequencies of the radial and vertical string loop oscillations are determined by the mass M and dimensionless spin a of the neutron star, and by dimensionless parameter ω describing combined effects of the string loop tension and its angular momentum. Equilibrium position of the string loop is given by its angular momentum and energy. The string-loop oscillation model can explain the observed kHz frequencies, but the stringy parameter ω cannot be the same for all the three HF QPO observations in the XTE J1701-462 source; the limits on the acceptable values of ω are given in dependence on the spacetime parameters M and a. However, the model implies restrict ion M > 3.3M on the neutron star mass that is too high to be compatible with the standard theory of neutron stars. A proper correction on the mass-limit can be generally introduced due to the electromagnetic interaction of an electrically charged string loop with magnetic field of the neutron star.