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William Zhang (Goddard Space Flight Center)
The frequency separation between the twin kHz QPOs in several low mass X-ray binaries has been found to vary. This variation has been usually assumed to mean that a simple beat frequency model, where the neutron star spin frequency and the higher one of the two QPO frequencies beat to generate the lower one, is inadequate to explain the phenomenon. In this paper I will present evidence from 4U 1636-53 that shows that, on the one hand, when both of the twin QPOs are strong and easily detectable, their separation in frequency is much less than 290 Hz which is presumably the neutron star spin frequency or half it according to studies of oscillations observed during several Type-I X-ray bursts; on the other hand, if one measures the separation using all the data, namely summing up all data segments (about 3000 seconds each) regardless of whether individually they show one or both or none of the QPO peaks, one finds that the separation is 290 Hz. This seems to indicate that the separation in frequency and the amplitudes of of the QPOs are correlated. When the separation is smaller than 290 Hz, the amplitudes are large and therefore easily detectable in individual data segments. When the separation is larger the amplitudes are weak and therefore the QPOs are not easily detectable in individual segments. I will discuss the implication of this observation on the simple beat frequency model.