Effects of Rapid Stellar Rotation on Equation‐of‐State Constraints Derived from Quasi‐periodic Brightness Oscillations

Quasi-periodic X-ray brightness oscillations (QPOs) with frequencies around akilohertz have now been discovered in more than a dozen neutron stars inlow-mass X-ray binary systems using the Rossi X-ray Timing Explorer. There isstrong evidence that the frequencies of the kilohertz oscillations are theorbital frequencies of accreting gas in nearly circular orbits around thesestars. Some stars that produce kilohertz QPOs may have spin frequencies greaterthan 400 Hertz. For spin rates this high, first-order analytic treatments ofthe effects of the star's rotation on its structure and the spacetime areinaccurate. Here we use the results of a large number of fully relativistic,self-consistent numerical calculations of the stellar structure of rapidlyrotating neutron stars and the interior and exterior spacetime to investigatethe constraints on the properties of such stars that can be derived if stablecircular orbits of various frequencies are observed. We have computed theequatorial radius of the star, the radius of the innermost stable circularorbit, and the frequency of the highest-frequency stable circular orbit asfunctions of the stellar spin rate, for spin rates up to the maximum possibleand for several illustrative equations of state. Our calculations show that theupper bounds on the stiffness of neutron star matter implied by a given orbitalfrequency are typically significantly stricter for stars with spin frequenciesmore than 400 Hertz than for slowly rotating stars.Comment: 14 pages plus three figures, uses aas2pp4.sty, submitted to The Astrophysical Journa