Abstract
In this paper, we improve the previous work on the MHD Alfvén 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 〈ρ〉 = 3M/(4πR 3 ) ≃ 2.4 × 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 ∼ 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 (∼ 1%M Edd ) are approximately ∼ 10 9 G and ∼ 10 8 G, respectively. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.