Implications of kHz quasi-periodic brightness oscillations in X-ray binaries for neutron star structure

Kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries (LM XBs) may represent the orbital frequencies of the innermost Keplerian orbit s around accreting neutron stars. Attempts have recently been made to deriv e constraints on the mass and the equation of state of the neutron star, by identifying the highest observed QPO frequency with the Keplerian frequenc y at the marginally stable orbit given by the general theory of relativity. These estimates have either neglected the effect of neutron star rotation or used an approximate treatment of rotation in general relativity. We rede rive these constraints using a fully general relativistic formalism includi ng the effect of rapid rotation. We also present constraints corresponding to the case where the innermost stable orbit touches the stellar surface.