Modeling the response of a standard accretion disc to stochastic viscous fluctuations

Abstract

The observed variability of X-ray binaries over a wide range of time-scales can be understood in the framework of a stochastic propagation model, where viscous fluctuations at different radii induce accretion rate variability that propagate inwards to the X-ray producing region. The scenario successfully explains the power spectra, the linear rms-flux relation as well as the time-lag between different energy photons. The predictions of this model have been obtained using approximate analytical solutions or empirically motivated models which take into account the effect of these propagating variability on the radiative process of complex accretion flows. Here, we study the variation of the accretion rate due to such viscous fluctuations using a hydro-dynamical code for the standard geometrically thin, gas pressure dominated $\alpha$-disc with a zero torque boundary condition. Our results confirm earlier findings that the time-lag between a perturbation and the resultant inner accretion rate variation depends on the frequency (or time-period) of the perturbation. Here we have quantified that the time-lag $t_{lag}\propto f^{-0.54}$, for time-periods less than the viscous time-scale of the perturbation radius and is nearly constant otherwise. This, coupled with radiative process would produce the observed frequency dependent time-lag between different energy bands. We also confirm that if there are random Gaussian fluctuations of the $\alpha$-parameter at different radii, the resultant inner accretion rate has a power spectrum which is a power-law.

17 pages, 9 figures

References in corpus (4)

• Modelling the behaviour of accretion flows in X-ray binaries
• Modelling the cross-spectral variability of the black hole binary MAXI J1659-152 with propagating accretion rate fluctuations
• Non-Linear Dynamics of Accretion Disks with Stochastic Viscosity
• The Effect of X-ray Irradiation on the Time Dependent Behaviour of Accretion Disks with Stochastic Perturbations

Cited by in corpus (3)

• A stochastic propagation model to the energy dependent rapid temporal behaviour of Cygnus X-1 as observed by AstroSat in the hard state
• Unveiling the temporal properties of MAXI J1820+070 through AstroSat observations
• Slim accretion disks: theory and observational consequences