as tr o - ph / 9 41 00 21 5 O ct 9 4 Accretion disc turbulence and the X - ray power spectra of black hole highstatesMichael

The high state of black hole candidates is characterized by a quasi-thermal emission component at kT 1 keV. In addition, this state tends to have very low variability which indicates that it is relatively stable, at least on short time scales. Most models of the high state imply that the bulk of the emission comes from an optically thick accretion disc; therefore, this state may be an excellent laboratory for testing our ideas about the physics of accretion discs. In this work we consider the implications of assuming that accretion disc viscosity arises from some form of turbulence. Speciically, we consider the simple case of three dimensional hydrodynamic turbulence. It is found that the coupling of such turbulence to acoustic modes in the disc can 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 parameters we can reproduce some of the high frequency power. We then brieey explore mechanisms for producing the low frequency power, and note the diiculty that a single variability mechanism has in reproducing the full range of observed variability. In addition, we outline ways in which future spacecraft missions { such as USA and XTE { can further constrain our model, especially at frequencies above 10 2 Hz. Black hole candidates (BHC) are typically identiied either from estimates of their mass, via a binary mass function, or by analogies with other BHC where one typically compares high energy emission and variability properties. (For a review of black hole candidates, including a summary of observed spectral and temporal properties, see Tanaka & Lewin 1994.) The energy spectra of BHC have been historically labelled based upon observations of the soft X-ray band (110 keV). Intense, quasi-thermal ux is referred to as the \high" state. Non-thermal ux (typically a power law with a photon index of 1:7) in this band indicates that the BHC is in the \oo" state, for extremely low intensity ux, or \low" state, for moderate intensity ux that in the 1 10 keV band is less than the high state ux in the same band. It is possible, however, for the bolometric luminosity to be greater in the low state than in the high state. See the comments by Tanaka …