Kilohertz Quasi-Periodic Oscillations, Magnetic Fields and Mass of Neutron Stars in Low-Mass X-Ray Binaries

It has recently been suggested that the maximum observed quasi-periodic oscillation (QPO) frequencies, $\nu_{max}$, for several low-mass X-ray binaries, particularly 4U 1820-30, correspond to the orbital frequency at the inner-most stable orbit of the accretion disk. This would imply that the neutron stars in these systems have masses $\go 2~M_\odot$, considerably larger than any well-measured neutron star mass. We suggest that the levelling off of $\nu_{QPO}$ may be also understood in terms of a steepening magnetic field which, although possibly dipolar at the stellar surface, is altered substantially by disk accretion, and presents a ``wall'' to the accretion flow that may be outside the innermost stable orbit. General relativistic effects add to the flattening of the $\nu_{QPO}-\mdot$ relation at frequencies below the Kepler frequency at the innermost stable orbit. We offer two other possible ways to reconcile the low value of $\nu_{max}$ ($\approx 1060$ Hz for 4U 1820-30) with a moderate neutron star mass, $\approx 1.4\msun$: at sufficiently large $\mdot$, either (i) the disk terminates in a very thin boundary layer near the neutron star surface, or (ii) $\nu_{QPO}$ is not the orbital frequency right at the inner edge of the disk, but rather at a somewhat larger radius, where the emissivity of the disk peaks.