Further investigation of MHD Alfven wave oscillation model for kHz QPOs

Based on the MHD Alfven wave oscillation model for the kHz quasi periodic oscillation (QPO), we investigate the comparisons between the model and the four sample sources, Sco X-1, 4U1608−52, 4U1735-44 and 4U1728−34, and obtained that the maximum upper and lower kHz QPO frequencies coincide and occur at about 1300 Hz. Moreover, the theoretical ratio between the maximum kHz QPO frequency and the maximum twin kHz QPO separation is about 3.2, which is close to the detected average value 3.4. Nonetheless, the neutron star (NS) mass-radius relation can be constrained by the simultaneously detected twin kHz QPO frequencies through the model. The implied NS magnetosphere corotation radii are homogeneous and about twice of star radii for the Z and Atoll sources, which results in the magnetic field strengths of stars in Z (Atoll) sources are about 10 (10) Gauss. Furthermore, some theoretical expectations on SAX J1808.4-3658 are proposed, such as, its highest kHz QPO frequency 833 Hz, which needs the observational confirmation.