Two-dimensional studies of inertial-acoustic oscillations in black hole accretion discs

Physics

Scientific paper

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Accretion, Accretion Discs, Black Hole Physics, Convection, Hydrodynamics, Instabilities, Waves

Scientific paper

Radiation hydrodynamic simulations of two-dimensional, optically thick, black hole accretion discs are performed under conditions of varying mass accretion rate, /M//M_Edd, and kinematic viscosity, nu. Specifically, the effects of inertial-acoustic oscillations and convection on disc structure and observable properties are studied. It is found that global oscillations with rms amplitudes of <~0.3 per cent can be present at the maximum epicyclic frequency in the disc. These oscillations are favoured for low accretion rates (/M<~/M_Edd) and large viscosities. Local oscillations at the local epicyclic frequencies are always present with rms amplitudes of <~0.2 per cent. These oscillations yield power spectra with spectral slopes of 0.98 and 1.33. Qualitatively similar behaviour is found for discs described by an alpha law or a constant nu. Oscillations may be present for a restricted range of parameters for any viscosity law in which the shear viscosity (mu=rhonu) increases upon compression, and they may be detectable with the X-ray Timing Explorer. It is also found that for /M>~0.1/M_Edd convection dominates the transfer of energy in the disc. The convection cells generally stretch from the mid-plane to the disc surface at low accretion rates (/M<~/M_Edd), while the cells appear on varying length-scales at high accretion rates (/M>~/M_Edd).

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