On the nature of the variability power decay toward soft spectral states in X-ray binaries: case study in Cygnus X-1.
TITARCHUK L. and SHAPOSHNIKOV N.
Abstract (from CDS):
A characteristic feature of the Fourier power density spectrum (PDS) observed from black hole X-ray binaries in low/hard and intermediate spectral states is a broadband-limited noise characterized by a constant below some frequency (a ``break'' frequency) and a power law above this frequency. It has been shown that the variability of this type can be produced by the inward diffusion of the local driving perturbations in a bounded configuration (accretion disk or corona). In the framework of this model, the perturbation diffusion time t0 is related to the phenomenological break frequency, while the PDS power-law slope above the ``break'' is determined by the viscosity distribution over the configuration. The perturbation diffusion scenario explains the decay of the power of X-ray variability observed in a number of compact sources (containing black holes and neutron stars) during an evolution of these sources from low/hard to high/soft states. We compare the model predictions with the subset of data from Cyg X-1 collected by the Rossi X-Ray Time Explorer (RXTE). Our extensive analysis of the Cyg X-1 PDSs demonstrates that the observed integrated power Px decreases approximately as the square root of the characteristic frequency of the driving oscillations νdr. The RXTE observations of Cyg X-1 allow us to infer Pdr and t0as a function of νdr. Using the inferred dependences of the integrated power of the driving oscillations Pdr and t0on νdrwe demonstrate that the power predicted by the model also decays as Px,diff∝ν–0.5dr, which is similar to the observed Pxbehavior. We also apply the basic parameters of observed PDSs, power-law indices, and low-frequency quasi-periodic oscillations to infer the Reynolds number (Re) from the observations using the method developed in our previous paper. Our analysis shows that Re increases from values of about 10 in low/hard state to about 70 during the high/soft state.