1999ApJ...525L.129T

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 kHz QPO, namely ν_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 ν_K. Such an oscillator has two branches characterized by a high frequency ν_h_ (∼1 kHz) and by a low frequency ν_L (∼50 Hz). The frequency ν_L_ depends strongly on the angle δ between the normal to the neutron star disk and Ω–the angular velocity of the magnetosphere surrounding the neutron star. In the lower part of the QPO spectrum (∼10 Hz), this model identifies the frequency of radial viscous oscillations ν_V (previously called ``extra noise component'') and the break frequency ν_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 ν_h_, ν_L, ν_b, and ν_V) have specific dependences on ν_K. This Letter focuses on the verification of the predicted relations. For the source 4U 1728-34, the best theoretical fit is obtained for δ=8°.3±1°.0, which is slightly larger than the value of δ=5°.5±0°.5 previously found for Scorpius X-1. In addition, we show that the theoretically derived power-law relation ν_b∝ν^1.61_V is consistent with the recent observations of other atoll and Z-sources.