We ascribe the twin kilohertz Quasi Periodic Oscillations ( kHz QPOs ) of X-ray spectra of Low Mass X-Ray Binaries ( LMXBs ) to the pseudo-Newtonian Keplerian frequency and the apogee and perigee precession frequency of the same matter in the inner disk , and ascribe 15 - 60 Hz QPO ( HBO ) to the apogee ( or perigee ) precession and its second harmonic frequency to both apogee and perigee precession in the outer disk boundary of the neutron star ( NS ) magnetosphere .
The radii of the inner and outer disks are correlated each other by a factor of two is assumed .
The obtained conclusions include : all QPO frequencies increase and frequency difference of twin kHz QPOs decreases with increasing the accretion rate .
The obtained theoretical relations between HBO frequency and twin kHz QPOs are simlilar to the measured empirical formula .
Further , the theoretical formula to calculate the NS mass by the twin kHz QPOs is proposed , and the resultant values are in the range of 1.4 to 1.8 M _ { \odot } .
QPOs from LMXBs likely provide an accurate laboratory for a strong gravitational field , by which a new method to determine the NS masses of LMXBs is suggested .