Testing the Transition Layer Model of Quasi‐periodic Oscillations in Neutron Star X‐Ray Binaries

We compare the theoretical predictions of the transition layer model withsome observational features of quasi-periodic oscillations (QPOs) in neutronstar X-ray binaries. We found that the correlation between horizontal branchoscillation (HBO) frequencies and kilohertz (kHz) QPO frequencies, thedifference between the low-frequency QPOs in atoll sources and HBOs in Zsources, and the correlation between the frequencies of low-frequency QPOs andbreak frequencies can be well explained by the transition layer model, providedthe neutron star mass is around 1.4 solar mass and the angle betweenmagnetosphere equator and accretion disk plane is around 6 degree. The observeddecrease of peak separation between two kHz QPO frequencies with the increaseof kHz QPO frequencies and the increase of QPO frequencies with the increase ofinferred mass accretion rate are also consistent with the theoreticalpredictions of transition layer model. In addition, we derive a simple equationthat can be adopted to estimate the angle ($\delta$) between magnetosphereequator and accretion disk plane by use of the simultaneously observed QPOfrequency data. We estimate these angles, in the range of 4 to 8 degrees, forfive Z sources and two atoll sources. The nearly constant $\delta$ value foreach source, derived from the different sets of simultaneously observed QPOfrequency data, provides a strong test of the theoretical model. Finally, wesuggest that the similar transition layer oscillations may be also responsiblefor the observed QPOs in accretion-powered millisecond X-ray pulsar andGalactic black hole candidates.Comment: 10 pages, 5 figures, to appear in ApJ, Vol. 55