The correlations between the spin frequencies and kHz QPOs of neutron stars in LMXBs

Aims.We studied the correlations between spin frequencies and kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries. Methods.The updated data on kHz QPOs and spin frequencies are statistically analyzed. Results. We find 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 higher than the spin frequency, i.e. $\nu_{\rm s}