The correlations between kHz and LF QPOs in the NSXBs

We study the correlations between low frequency (LF) and kHz quasi-periodic oscillations (QPOs) for 20 neutron star X-ray bianaries (NSXBs). We find that both upper and lower kHz QPOs correlate with the LF QPOs. However, the correlations show 5 parallel branches in the kHz QPO frequency vs. LF QPO frequency relation diagram. We notice that the source will jump from one parallel branch to another when different harmonic frequency of the LF QPO is used to plot the correlation between kHz and LF QPOs. We infer that the parallel branches maybe due to the multi-harmonic QPOs in the NSXBs.