Maximum orbital frequencies for rotating neutron stars and strange stars and kHz QPOs in low-mass X-ray binaries

Abstract

With the discovery of kilo‐hertz quasi‐periodic oscillations (QPOs) in the X‐ray fluxes from accreting neutron star X‐ray Binaries, the Rossi X‐ray Timing Explorer (RXTE) has demonstrated that fast X‐ray timing has the potential to measure the motion of matter in strong gravity fields and to constrain masses and radii of neutron stars. The spin‐up of a neutron star crucially depends on the maximum orbital frequency around it, as do a host of other high energy accretion phenomena in low mass X‐ray binaries, including quasi‐periodic oscillations (QPOs) in the X‐ray flux. Here we summarize our studies on rotational and orbital frequencies for uniformly rotating neutron stars and strange stars. The calculation are performed for both normal and supramassive evolutionary sequences of compact stars rotating at any possible rates. It is possible to discriminate between strange quark stars and neutron stars in some theories of kHz QPOs.