Innermost stable circular orbits around strange stars and kHz QPOs in low-mass X-ray binaries

Exact calculations of innermost stable circular orbit (ISCO) around rotatingstrange stars are performed within the framework of general relativity.Equations of state (EOS) of strange quark matter based on the MIT Bag Modelwith massive strange quarks and lowest order QCD interactions, are used. Thepresence of a solid crust of normal matter on rotating, mass accreting strangestars in LMXBs is taken into account. It is found that, contrary to neutronstars, above some minimum mass a gap always separates the ISCO and stellarsurface, independently of the strange star rotation rate. For a given baryonmass of strange star, we calculate the ISCO frequency as function of stellarrotation frequency, from static to Keplerian configuration. For masses close tothe maximum mass of static configurations the ISCO frequencies for static andKeplerian configurations are similar. However, for masses significantly lowerthan the maximum mass of static configurations, the minimum value of the ISCOfrequency is reached in the Keplerian limit. Presence of a solid crustincreases the ISCO frequency for the Keplerian configuration by about tenpercent compared to that for a bare strange star of the same mass