We have calculated exact models of rotating strange stars - these computations are in excellent agreement (Table 2) with the very recent results obtained by a highly accurate code based on spectral methods (Gourgoulhon et al., 1999). We found the scalings of M, R, with the parameters a, and in the equation of state of self-bound quark matter . In addition, we calculate the innermost stable orbits and find that, unlike for static models, for strange stars rotating at the equatorial mass-shedding limit the orbital frequencies can extend to values below kHz. For the same models the radius of the ISCO is about 11% larger than the circumferential stellar radius, independently of the central density, or the value of .
Table 2. Comparison of two codes.
Our results show that the highest observed QPO frequencies in low-mass X-ray binaries (such as 1.07 kHz in 4U 1820-30) could be the orbital frequencies in the innermost stable circular orbit about strange stars, if only the stars are rotating sufficiently rapidly (as is expected in these old accreting sources). Thus, the compact objects in LMXBs could, in principle, be rapidly rotating strange stars. Further conclusions about the nature of LMXBs and of the kHz QPOs would be possible, if either the mass or the rotational period of the accreting stellar remnant were known.
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999