Universal relations for the Keplerian sequence of rotating neutron stars

We investigate the Keplerian (mass-shedding) sequence of rotating neutron stars. Twelve different equations of state are used to describe the nuclear structure. We find four fitting relations which connect the rotating frequency, mass and radius of stars in the mass-shedding limit to the mass and radius of stars in the static sequence. We show the breakdown of approximate relation for the Keplerian frequency derived by Lattimer and Prakash [Science 304, 536 (2004)] and then we present a new, equation of state (EOS)-independent and more accurate relation. This relation fits the Keplerian frequency of rotating neutron stars to about 2% for a large range of the compactness MS/RS of the reference nonrotating neutron star, namely the static star with the same central density as the rotating one. The performance of the fitting formula is close to 4% for MS/RS≤0.05 M⊙/km (fK≤350 Hz). We present additional EOS-independent relations for the Keplerian sequence including relations for MKfK and RKfK in terms of MSfS and RSfS, respectively, one of MK/RK as a function of fK/fS and MS/RS, and a relation between the MK, RK and fK. These new fitting relations are approximately EOS independent with an error in the worst case of 8%. The universality of the Keplerian sequence properties presented here add to the set of other neutron star universal relations in the literature such as the I-Love-Q relation, the gravitational binding energy and the energy, angular momentum and radius of the last circular orbit of a test particle around rotating neutron stars. This set of universal, analytic formulas facilitates the inclusion of general relativistic effects in the description of relativistic astrophysical systems involving fast rotating neutron stars.