The Highest Dynamical Frequency in the Inner Region of an Accretion Disk

Abstract

In the inner regions of accretion disks 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 kappa is higher than the azimuthal frequency Omega. This has significant implications for models of the twin kHz QPOs observed in many neutron-star sources that traditionally identify the frequencies of the two kHz QPOs with dynamical frequencies in the accretion disk. The recognition that the highest frequency in the transition or boundary region of the disk is actually the epicyclic frequency also modifies significantly the constraints imposed by the observation of high-frequency QPOs on the mass and radius of the compact objects.