Effects of rapid stellar rotation on equation-of-state constraints derivedfrom quasi-periodic brightness oscillations

Quasi-periodic X-ray brightness oscillations (QPOs) with frequencies greate r than or similar to 1 kHz have now been discovered in more than a dozen ne utron stars in low-mass X-ray binary systems using the Rossi X-Ray Timing E xplorer. There is strong evidence that the frequencies of some kilohertz os cillations are the orbital frequencies of accreting gas in nearly circular orbits around these stars. Some stars that produce kilohertz QPOs may have spin frequencies greater than or similar to 400 Hz. For spin rates this hig h, first-order analytic treatments of the effects of the star's rotation on its structure and the spacetime are inaccurate. Here we use the results of a large number of fully relativistic, self-consistent numerical calculatio ns of the stellar structure of rapidly rotating neutron stars and their int erior and exterior spacetimes to investigate the constraints on the propert ies of such stars that can be derived if stable circular orbits of various frequencies are observed. We have computed the equatorial radius of the sta r, the radius of the innermost stable circular orbit, and the frequency of the highest frequency stable circular orbit as functions of the stellar spi n rate for spin rates up to the maximum possible and for several illustrati ve equations of state. Our calculations show that the upper bounds on the s tiffness of neutron star matter implied by a given orbital frequency are ty pically significantly stricter for stars with spin frequencies greater than or similar to 400 Hz than for slowly rotating stars.