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**Session 68 - Degenerate Stars, Pulsars.**

*Display session, Thursday, June 11*

*Atlas Ballroom, *

## [68.11] Effects of Rapid Stellar Spin on Equation of State Constraints Derived from QPOs

*F. K. Lamb (U. Illinois), M. C. Miller (U. Chicago), G. Cook (Cornell)*
Quasi-periodic X-ray brightness oscillations (QPOs) with
frequencies \sim1 kHz have now been discovered in more
than a dozen neutron stars in low-mass X-ray binary systems
using the Rossi X-ray Timing Explorer. There is strong
evidence that the frequencies of these oscillations 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 400 Hz. For
spin rates this high, 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
calculations of the stellar structure of rapidly rotating
neutron stars and the interior and exterior spacetime to
investigate the constraints on the properties of such stars
that can be derived if stable circular orbits of various
frequencies are observed. We have computed the equatorial
radius of the star, the radius of the innermost stable
circular orbit, and the frequency of the highest-frequency
stable circular orbit as functions of the stellar spin rate,
for spin rates up to the maximum possible and for several
illustrative equations of state. Our calculations show that
the constraints on the equation of state of neutron star
matter implied by a given orbital frequency can be
significantly stricter for stars with spin frequencies
greater than 400 Hz than for slowly rotating stars.

**Program
listing for Thursday**