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=8.3^0+/-1.0d, which is slightly larger than the value of δ=5.5^o+/-0.5d previously found for Scorpius X-1. In addition, we show that the theoretically derived power-law relation v_b~nu{^1.61}_V is consistent with the recent observations of other atoll and Z-sources.

Timing Spectroscopy of Quasi-Periodic Oscillations in the Low-Mass X-Ray Neutron Star Binaries

TITARCHUK, Lev;
1999

Abstract

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=8.3^0+/-1.0d, which is slightly larger than the value of δ=5.5^o+/-0.5d previously found for Scorpius X-1. In addition, we show that the theoretically derived power-law relation v_b~nu{^1.61}_V is consistent with the recent observations of other atoll and Z-sources.
Titarchuk, Lev; Osherovich, V.; Kuznetsov, S.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/532951
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