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We analyze the relation between the emission radii of twin kilohertzquasi-periodic oscillations (kHz QPOs) and the co-rotation radii of the 12neutron star low mass X-ray binaries (NS-LMXBs) which are simultaneouslydetected with the twin kHz QPOs and NS spins. We find that the averageco-rotation radius of these sources is r_co about 32 km, and all the emissionpositions of twin kHz QPOs lie inside the corotation radii, indicating that thetwin kHz QPOs are formed in the spin-up process. It is noticed that the upperfrequency of twin kHz QPOs is higher than NS spin frequency by > 10%, which mayaccount for a critical velocity difference between the Keplerian motion ofaccretion matter and NS spin that is corresponding to the production of twinkHz QPOs. In addition, we also find that about 83% of twin kHz QPOs clusteraround the radius range of 15-20 km, which may be affected by the hard surfaceor the local strong magnetic field of NS. As a special case, SAX J1808.4-3658shows the larger emission radii of twin kHz QPOs of r about 21-24 km, which maybe due to its low accretion rate or small measured NS mass (< 1.4 solar mass).

Probing the accretion disc structure by the twin kHz QPOs and spins of neutron stars in LMXBs