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 \mathrel { \raise 1.29 pt \hbox { $ > $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 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 } - \dot { M } 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 M _ { \odot } : at sufficiently large \dot { M } , 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 .