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

Lev Titarchuk

Vladimir Osherovich

Sergey Kuznetzov

Abstract

Precise measurements of the frequencies of the two simultaneous kiloHertz quasiperiodic oscillations (kHz QPOs), nK and nh, in the atoll source 4U 1728-34 carried out by Mendez & van der Klis (1999) cast additional doubts about the validity of the simple beat-frequency interpretation and some of the modifications introduced to explain the results of the varying Dn = nh-nK. A new model (Osherovich and Titarchuk 1999, Titarchuk and Osherovich 1999) explains the variation of Dn suggesting that nh is the upper hybrid frequency of the Keplerian oscillator under the influence of the Coriolis force. Such an oscillator has two branches characterized with high frequency nh and with low frequency nL. The frequency nL depends strongly on the angle d between the normal to the neutron star disk and W - the angular velocity of the magnetosphere surrounding the neutron star. In the lower part of the QPO spectrum, this model identifies the frequency of radial viscous oscillations nv and the break frequency nb, which is associated with the diffusive process in the transition region (the innermost part of the disk). According to this model, all frequencies (namely nh, nL, nb and nv) have specific dependences on nK. This paper focuses on the verification of the predicted relations. For two sources, namely Sco X-1 and 4U 1728-34, we present a comprehensive classification of QPO within the framework of this model. For the source 4U 1728-34, the best theoretical fit is obtained for d = 8.31o, which is slightly larger than d = 5.50.5o previously found for Sco X-1. In addition, we verify the theoretically derived power law relation nb nv1.61 using recent observations of other atoll and Z-sources.


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