Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations

Details Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21333804l&link_type=abstract

American Astronomical Society, AAS Meeting #213, #338.04; Bulletin of the American Astronomical Society, Vol. 41, p.400

Astronomy and Astrophysics

Astronomy

Scientific paper

We study the global stability of non-axisymmetric p-modes (also called inertial-acoustic modes) trapped in the inner-most regions of
accretion discs around black holes. We show that the lowest-order (highest-frequency) p-modes, with frequencies ω=(0.5-0.7) mΩISCO (where $m=1,2,3,\dots, ΩISCO is the disc rotation frequency at the Inner-most Stable Circular Orbit, ISCO), can be overstable due to general relativistic effects, according to which the radial epicyclic frequency κ is a non-monotonic function of radius near the black hole. The mode is trapped inside the corotation resonance radius rc and carries a
negative energy. The mode growth arises primarily from wave absorption at the corotation resonance, and the sign of the wave absorption depends on the gradient of the disc vortensity, ζ=κ2/(2ΩΣ) (where Σ is the surface density). The mode growth is further enhanced by wave transmission beyond the corotation barrier. We also study how the rapid radial inflow at the inner edge of the disc affects the mode trapping and growth. Our analysis of the behavior of the fluid perturbations in the transonic flow near the ISCO indicates that, while the inflow tends to damp the mode, the damping effect is sufficiently small under some conditions so that net mode growth can still be achieved. Overstable non-axisymmetric p-modes driven by the corotational instability provide a plausible explanation for the high-frequency (> 100 Hz) quasi-periodic oscillations (HFQPOs) observed from a number of black-hole X-ray binaries in the very high state. The absence of HFQPOs in the soft (thermal) state may result from mode damping due to the radial infall at the ISCO.

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