On the nature of the variability power decay towards soft spectral states in X-ray binaries. Case study in Cyg X-1

TL;DR

This paper investigates the decay of variability power in X-ray binaries, specifically Cyg X-1, using a diffusion model to explain how the power spectrum changes with spectral states, supported by RXTE data analysis.

Contribution

It introduces a diffusion-based model linking variability power decay to spectral state changes in X-ray binaries, validated with extensive RXTE observations of Cyg X-1.

Findings

Observed power decreases as the square root of the driving oscillation frequency.

Model predictions match the observed decay of variability power.

Inferred diffusion timescales correlate with spectral state transitions.

Abstract

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 broad band-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 t_0 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. he perturbation diffusion scenario explains the decay of the power of X-ray variability observed in a number of compact sources (containing black hole and neutron star) during an evolution of theses 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 P_x decreases approximately as a square root of the characteristic frequency of the driving oscillations \nu_{dr}. The RXTE observations of Cyg X-1 allow us to infer P_{dr} and t_0 as a function of \nu_{dr}. Using the inferred dependences of the integrated power of the driving oscillations P_{dr} and t_0 on \nu_{dr} we demonstrate that the power predicted by the model also decays as P_{x,diff} proportional to \nu_{dr}^{-0.5} that is similar to the observed P_{x} behavior.