Power spectra of black holes and neutron stars as a probe of hydrodynamical structure of the source. Diffusion theory and its application to Cyg X-1 and Cyg X-2 X-ray observations

摘要

We present a model of Fourier Power Density Spectrum (PDS) formation inaccretion powered X-ray binary systems derived from the first principles of thediffusion theory. Timing properties of X-ray emission are considered to be aresult of diffusive propagation of the driving perturbations in a boundedmedium. We prove that the integrated power P_x of the resulting PDS is only asmall fraction of the integrated power P_dr of the driving oscillations, whichis distributed over the disk. The resulting PDS continuum is a sum of twocomponents, a low frequency (LF) component which presumably originates in anextended accretion disk and a high frequency (HF) component which originates inthe innermost part of the source (Compton cloud or corona). The LF PDScomponent has a power-law shape with index of 1.0-1.5 at higher frequencies(``red'' noise) and a flat spectrum below a characteristic (break) frequency(``white'' noise). This white-red noise (WRN) continuum spectrum holdsinformation about the physical parameters of the bounded extended medium,diffusion time scale and the dependence law of viscosity vs radius. We applyour model of the PDS to a sample of RXTE and EXOSAT timing data from Cyg X-1and Cyg X-2 which describes adequately the spectral transitions in thesesources.