Abstract The widely used expression of the radial epicyclic frequency, which is important in the relativistic precession model to interpret the observed quasi-periodic oscillation(QPO) in X-ray binaries, is derived by considering the perturbation of a circular-orbiting test particle around a compact object. However, this is not consistent with the real configuration of an accretion disk. Here we obtain the expressions of the radial epicyclic frequency of a perfect fluid disk in both Schwarzschild and Kerr metrics and find that they are different from the previous expressions if the pressure of fluid is considered. The effects of these new expressions on the theoretical QPO models are discussed.
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