The correlations between the spin frequencies and kHz QPOs of Neutron Stars in LMXBs

TL;DR

This study analyzes the relationship between neutron star spin frequencies and kHz QPOs in LMXBs, revealing minimum frequency ratios and an anti-correlation between QPO peak separation and spin frequency, supported by updated observational data.

Contribution

It provides new statistical insights into the correlation patterns between spin frequencies and kHz QPOs, including a shifted correlation model for specific pulsars.

Findings

Minimum upper kHz QPO frequency is at least 1.3 times the spin frequency.

Average peak separation anti-correlates with spin frequency in Atoll sources.

Shifted correlation explains data for certain millisecond X-ray pulsars.

Abstract

We studied the correlations between spin frequencies and kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low mass X-ray binaries. The updated data of kHz QPOs and spin frequencies are statistically analyzed. We found that when two simultaneous kHz QPOs are present in the power spectrum, the minimum frequency of upper kHz QPO is at least 1.3 times larger than the spin frequency, i.e. \nu_{s}<\nu_{2min}/1.3. We also found that the average kHz QPO peak separation in 6 Atoll sources anti-correlates with the spin frequency in the form \lan\dn\ran = -(0.19\pm0.05)\ns+(389.40\pm21.67)Hz. If we shifted this correlation in the direction of the peak separation by a factor of 1.5, this correlation matches the data points of the two accretion powered millisecond X-ray pulsars, SAX J1808.4-3658 and XTE J1807-294.