Broad-band variability in accreting compact objects

TL;DR

This paper compares the broad-band variability observed in accreting compact objects like X-ray binaries and cataclysmic variables, highlighting their similarities and discussing the fluctuating accretion disk model as an explanation.

Contribution

It provides a comparative analysis of variability properties across different accreting systems and discusses the applicability of the fluctuating accretion disk model.

Findings

Similar power spectral density shapes in XRBs and CVs

Consistent rms-flux relations across systems

Fourier-dependent time lags observed in both types

Abstract

Cataclysmic variable stars are in many ways similar to X-ray binaries. Both types of systems possess an accretion disk, which in most cases can reach the surface (or event horizon) of the central compact object. The main difference is that the embedded gravitational potential well in X-ray binaries is much deeper than those found in cataclysmic variables. As a result, X-ray binaries emit most of their radiation at X-ray wavelengths, as opposed to cataclysmic variables which emit mostly at optical/ultraviolet wavelengths. Both types of systems display aperiodic broad-band variability which can be associated to the accretion disk. Here, the properties of the observed X-ray variability in XRBs are compared to those observed at optical wavelengths in CVs. In most cases the variability properties of both types of systems are qualitatively similar once the relevant timescales associated with the inner accretion disk regions have been taken into account. The similarities include the observed power spectral density shapes, the rms-flux relation as well as Fourier-dependant time lags. Here a brief overview on these similarities is given, placing them in the context of the fluctuating accretion disk model which seeks to reproduce the observed variability.