Excitation of low-frequency QPOs in black hole accretion

¹ Max Planck Institute for Astrophysics, Box 1317, 85741 Garching, Germany e-mail: dgiannios@physics.uoc.gr
² University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece
³ Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece

Received: 16 July 2003
Accepted: 2 July 2004

Abstract

We study possible mechanisms of excitation of quasiperiodic oscillations in the accretion flow of black hole accreters in their hard spectral states, in the context of the “truncated disk” model. Quasi-spherical oscillations of the inner ion-supported accretion flow (ISAF) can be excited by the interaction of this hot flow with the cool disk extending outward from it. The fundamental mode of (p-mode) oscillation is most easily excited, and has a frequency near the Kepler frequency at the inner edge of the cool disk. The strongest excitation mechanism is a feedback loop involving cooling of the ISAF by soft photons from the cool disk and heating of the cool disk by the ISAF, while synchrotron emission can be a relatively strong damping effect. Growth times are computed by detailed Comptonization calculations of the interaction of an idealized ISAF with a cool disk. Typical growth times as short as a few dynamical times are found, while amplitudes can be as large as 10%.