ACCRETION DISC TURBULENCE AND THE X-RAY POWER SPECTRA OF BLACK-HOLE HIGH STATES

The high state of black hole candidates is characterized by a quasi-th ermal emission component at kT similar to 1 keV. In addition, this sta te tends to have very low variability, which indicates that it is rela tively stable, at least on short time-scales. Most models of the high state imply that the bulk of the emission comes from an optically thic k accretion disc; therefore this state may be an excellent laboratory for testing our ideas about the physics of accretion discs. In this wo rk, we consider the implications of assuming that accretion disc visco sity arises from some form of turbulence. Specifically, we consider th e simple case of three-dimensional hydrodynamic turbulence. It is foun d that the coupling of such turbulence to acoustic modes in the disc c an alter the disc emission. We calculate the amplitude and frequencies of this modulation, and we express our results in terms of the X-ray power spectral density. We compare our calculations with observations of the black hole candidate GS 1124-683, and show that for certain par ameters we can reproduce some of the high-frequency power. We then bri efly explore mechanisms for producing the low-frequency power, and not e the difficulty that a single variability mechanism has in reproducin g the full range of observed variability. In addition, we outline ways in which future spacecraft missions - such as USA and XTE - can furth er constrain our model, especially at frequencies above similar to 10( 2) Hz.