Strong-Field General Relativity and Quasi-periodic Oscillations in X-Ray Binaries

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Abstract
Quasi-periodic oscillations (QPOs) at frequencies near 1000 Hz were recently discovered in several X-ray binaries containing neutron stars. Two sources show no correlation between QPO frequency and source count rate. We suggest that the QPO frequency is determined by the Keplerian orbital frequency near the marginally stable orbit predicted by general relativity in strong gravitational fields. The radius of the marginally stable orbit does not depend on the luminosity or mass accretion rate of the source; therefore the QPO frequency does not depend on the source count rate. The QPO frequencies observed from 4U 1636-536 imply that the mass of the neutron star is 2.02 ± 0.12 M☉. Interpretation of the 4.1 keV absorption line observed from 4U 1636-536 as due to Fe XXV ions then implies a neutron star radius of 9.6 ± 0.6 km.
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