Chinese Acad Sci, Natl Astron Observ, Beijing 100012, Peoples R China
; Natl Tsing Hua Univ, Dept Phys, Hsinchu 30013, Taiwan
; Natl Tsing Hua Univ, Inst Astron, Hsinchu 30013, Taiwan
; Chinese Acad Sci, Inst High Energy Phys, Astron Inst, Beijing 100039, Peoples R China
In this paper, we improve the previous work on the MHD Alfven wave oscillation model for the neutron star (NS) kHz quasi-periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X-ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as < p > = 3M/(4 pi rR(3)) similar or equal to 2.4 x 10(14) (A/0.7)(2) g/cm(3), and we also compare the M-R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4-3658 are mentioned, such as its highest kHz QPO frequency similar to 870 Hz, which is about 1.4-1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate M(Edd)) and Atoll sources (similar to 1% M(Edd)) are approximately similar to 10(9) G and similar to 10(8) G, respectively. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Wcinheim.