Astron. Astrophys. 352, L116-L120 (1999)

Letter to the Editor

Keplerian frequencies and innermost stable circular orbits of rapidly rotating strange stars

N. Stergioulas ^1,2, W. Kluniak ^3,4 and T. Bulik <sup>3

1</sup> Max-Planck-Institut für Gravitationsphysik, Am Mühlenberg 5, 14476 Golm, Germany
² Aristotle University of Thessaloniki, Department of Physics, Thessaloniki 54006, Greece
³ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warszawa, Poland
⁴ University of California, Institute for Theoretical Physics, Santa Barbara, CA 93106, USA

Received 9 September 1999 / Accepted 2 November 1999

Abstract

It has been suggested that the frequency in the co-rotating innermost stable circular orbit (ISCO) about a compact stellar remnant can be determined through X-ray observations of low-mass X-ray binaries, and that its value can be used to constrain the equation of state of ultradense matter. Upon constructing numerical models of rapidly rotating strange (quark) stars in general relativity, we find that for stars rotating at the equatorial mass-shedding limit, the ISCO is indeed above the stellar surface, for a wide range of central energy densities at a height equal to 11% of the circumferential stellar radius, which scales inversely with the square root of the energy density, , of self-bound quark matter at zero presure. For these models, the ISCO frequency is % of the stellar rotational frequency, whose maximum value is attained for a model close to the maximum-mass model, with . In contrast to static models, ISCO frequencies below 1.1 kHz are allowed - in fact, at the canonical value , the ISCO frequencies of rapidly rotating strange stars can be as low as 0.9 kHz for a strange star. Hence, the presence of strange stars in low-mass X-ray binaries cannot be excluded on the basis of the currently observed frequencies of kHz QPOs, such as the cut-off frequency of 1066 Hz in 4U 1820-30.

Key words: dense matter stars: neutron equation of state X-rays: stars

Send offprint requests to: W. Kluniak

© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999

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