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Astron. Astrophys. 344, L71-L74 (1999)

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Letter to the Editor

Do LMXBs contain strange stars?

Tomasz Bulik 1, Dorota Gondek-Rosiska 1 and Wodzimierz Kluniak 1,2

1 Nicolaus Copernicus Astronomical Center, Bartycka 18, PL-00-716 Warszawa, Poland
2 Department of Physics, University of Wisconsin, Madison, WI 53706, USA

Received 11 February 1999 / Accepted 9 March 1999


It has been suggested that low mass X-ray binaries (LMXBs) contain strange (quark) stars. We show that the properties of bulk strange matter are severely constrained if the recently discovered kilohertz quasi periodic oscillations (QPOs) of the X-ray flux from LMXBs occur at orbital frequencies around strange stars. In particular, the simplest equation of state of strange matter is ruled out if the observed saturation (at 1.07 kHz) of the QPO frequency in 4U 1820-30 occurs at the orbital frequency either just outside the surface of the strange star or in the innermost (marginally) stable orbit allowed by general relativity.

Key words: dense matter – equation of state – stars: binaries: general – X-rays: stars

© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999