Statistical properties of twin kHz QPO in neutron star LMXBs

TL;DR

This study analyzes the distribution and properties of twin kHz QPOs in neutron star LMXBs, revealing shared characteristics across source types and potential correlations with neutron star spin frequencies.

Contribution

It provides a comprehensive analysis of twin kHz QPOs in 26 neutron star LMXBs, highlighting differences and similarities between Atoll and Z sources, and explores their relation to neutron star spin.

Findings

Atoll and Z sources show different u_1, u_2, and u_2/ u_1 distributions.

The distribution of abla u is not constant, indicating variable QPO frequency differences.

NS spin frequency distribution is higher than that of radio MSPs, suggesting observational biases.

Abstract

We collect the data of twin kilohertz quasi-periodic oscillations (kHz QPOs) published before 2012 from 26 neutron star (NS) low-mass X-ray binary (LMXB) sources, then 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 (about 408Hz) than that (about 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 kHz QPOs and NS spin frequency \nu_s, we find the approximate correlations of the mean values of \Delta\nu with NS spin and its half, respectively.