Constraints on Neutron Star Masses and Radii from Kilohertz QPOs

The frequencies of the highest-frequency kilohertz QPOs recently discovered in some sixteen neutron stars in low-mass X-ray binary systems are most likely orbital frequencies. If so, these QPOs provide tight upper bounds on the masses and radii of these neutron stars and interesting new constraints on the equation of state of neutron star matter. If the frequency of a kilohertz QPO can be established as the orbital frequency of the innermost stable circular orbit, this would confirm one of the key predictions of general relativity in the strong-field regime. If the spin frequency of the neutron star can also be determined, the frequency of the QPO would fix the mass of the neutron star for each assumed equation of state. Here we describe how bounds on the stellar mass and radius can be derived and how these bounds are affected by the stellar spin. We also discuss detection of the innermost stable circular orbit.