Kilohertz quasi-periodic oscillations ln low-mass X-ray binary sources andtheir relation to the neutron star magnetic field

Starting from the observation that kilohertz quasi-periodic oscillations (k Hz QPOs) occur in a very narrow range of X-ray luminosities in neutron star low-mass X-ray binaries, we try to link the kHz QPO observability to varia tions of the neutron star magnetospheric radius, in response to changing ma ss inflow rate. At low luminosities, the drop-off of kHz QPO activity may b e explained by the onset of the centrifugal barrier, when the magnetospheri c radius reaches the corotation radius. At the opposite side, at higher lum inosities, the magnetospheric radius may reach the neutron star and the van ishing of the magnetosphere may lead to the stopping of the kHz QPO activit y. If we apply these constraints, the magnetic fields of atoll [B similar t o (0.3-1) x 10(8) G for Aq1 X-1] and Z [B similar to (1-8) x 10(8) G for Cy g X-2] sources can be derived. These limits naturally apply in the framewor k of beat-frequency models but can also work in the case of general relativ istic models.