Spectroscopy of Quasi-Periodic Oscillations in the Low-Mass
X-ray Neutron Star Binaries
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 = 8.3±1o, which is slightly larger than
d = 5.5±0.5o previously found for Sco X-1. In addition, we
verify the theoretically derived power law relation nb µ nv1.61 using recent observations of other atoll and Z-sources.
File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:19.