The highest dynamical frequency in the inner region of an accretion disc

Abstract

In the inner regions of accretion discs around compact objects, the orbital frequency of the gas deviates from the local Keplerian value. For long-wavelength modes in this region, the radial epicyclic frequency K is higher than the azimuthal frequency Ω. This has significant implications for models of the twin kilohertz quasi-periodic oscillations (QPOs) observed in many neutron star sources that traditionally identify the frequencies of the 2 kHz QPOs with dynamical frequencies in the accretion disc. The recognition that the highest frequency in the transition or boundary region of the disc may be the epicyclic frequency also significantly affects the constraints imposed by the observation of high-frequency QPOs on the mass and radius of the compact objects.