Rotating Skyrmion Stars

In a previous paper, using an equation of state of dense matter representing a fluid of Skyrmions we constructed the corresponding non-rotating compact-star models in hydrostatic equilibrium; these are mostly fluid stars (the Skyrmion fluid) thus naming them Skyrmion Stars. Here we generalize our previous calculations by constructing equilibrium sequences of rotating Skyrmion stars in general relativity using the computer code RNS developed by Stergioulas. We calculated their masses and radii to be 0.4 ≤ M/M⊙ ≤ 3.45, and 13.0 km ≤ R ≤ 23.0 km, respectively (R being the circumferential radius of the star). The period of the maximally rotating Skyrmion stars is calculated to be 0.8 ms ≤ P ≤ 2.0 ms. We find that a gap (the height between the star surface and the inner stable circular orbit) starts to appear for M ∼ 2.0M⊙. Specifically, the Skyrmion star mass range with an existing gap is calculated to be 1.8 < M/M⊙ < 3.0 with the corresponding orbital frequency 0.8 kHz < νISCO < 1.3 kHz. We apply our model to the 4U 1820-30 low mass X-ray binary and suggest a plausible Skyrmion star candidate in the 4U 1636-53 system. We discuss the difficulties encountered by our model in the 4U 0614+09 case with the highest known Quasi-Periodic Oscillation frequency of 1329 Hz. A comparative study of Skyrmion stars and models of neutron stars based on recent/modern equations of state is also presented.