Timing spectroscopy of quasi-periodic oscillations in the low-mass X-ray neutron star binaries

Precise simultaneous measurements of the frequencies of the two kilohertz quasi-periodic oscillations (referred in the literature as upper and lower kHz QPOs) cast doubts on the validity of the simple beat-frequency interpretation and some of the modifications introduced to explain the results of the varying frequency difference. A new model explains the variation of the frequency difference, suggesting that the upper Wt QPO, namely v(h), is an upper hybrid frequency of the Keplerian oscillator under the influence of the Coriolis force and the lower kHz QPO is the Keplerian frequency v(K). Such an oscillator has two branches characterized by a high frequency v(h) (similar to 1 kHz) and by a low frequency v(L) (similar to 50 Hz). The frequency v(L) depends strongly on the angle delta between the normal to the neutron star disk and Omega-the angular velocity of the magnetosphere surrounding the neutron star In the lower part of the QPO spectrum (similar to 10 Hz), this model identifies the frequency of radial viscous oscillations v(V) (previously called ''extra noise component") and the break frequency v(b), which is associated with the diffusive process in the transition region (the innermost part of the disk). According to this model, all frequencies (namely v(h), y(L), y(b), and v(V)) have specific dependences on v,. This Letter focuses on the verification of the predicted relations. For the source 4U 1728-34, the best theoretical fit is obtained for delta = 8 degrees.3 +/- 1 degrees.0, which is slightly larger than the value of delta = 5 degrees.5 +/- 0 degrees.5 previously found for Scorpius X-1. In addition, we show that the theoretically derived power-law relation v(b) proportional to v(V)(1.61) is consistent with the recent observations of other atoll and Z-sources.