D. H. Wang, L. Chen, C. M. Zhang, Y. J. Lei, J. L. Qu
With 26 twin kilohertz quasi-periodic oscillation (kHz QPO) sources in neutron star (NS) low-mass X-ray binaries (LMXBs) published before 2012, we analyze the centroid frequency ({\nu}) distribution of twin kHz QPOs (lower frequency {\nu}1 and upper frequency {\nu}2) both for Atoll and Z sources. For the data without shift-and- add, we find that Atoll and Z sources show the different distributions of {\nu}1, {\nu}2 and {\nu}2/{\nu}1, but the same distribution of \Delta{\nu} (difference of twin kHz QPOs), which indicates that twin kHz QPOs may share the common properties of LXMBs and have the same physical origins. The distribution of \Delta{\nu} is quite different from constant value, so is {\nu}2/{\nu}1 from constant ratio. The weighted mean values and maxima of {\nu}1 and {\nu}2 in Atoll sources are slightly higher than those in Z sources.We also find that shift-and-add technique can reconstruct the distribution of {\nu}1 and \Delta{\nu}. The K-S test results of {\nu}1 and \Delta{\nu} between Atoll and Z sources from data with shift-and-add are quite different from those without it, and we think that this may be caused by the selection biases of the sample. We also study the properties of the quality factor (Q) and the root-mean-squared (rms) amplitude of 4U 0614+09 with the data from the two observational methods, but the errors are too big to make a robust conclusion. The NS spin frequency ({\nu}s) distribution of 28 NS-LMXBs show a bigger mean value (\sim 408 Hz) than that (\sim 281 Hz) of the radio binary millisecond pulsars (MSPs), which may be due to the lack of the spin detections from Z sources (systematically lower than 281 Hz). Furthermore, on the relations between the spin frequency {\nu}s and kHz QPOs, there is no obvious correlation between \Delta{\nu} and {\nu}s.
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http://arxiv.org/abs/1301.5478
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