Investigating the Theory of Propagating Fluctuations with Numerical Models of Stochastic Accretion Discs

TL;DR

This study uses numerical simulations of stochastic accretion discs to investigate propagating fluctuations, revealing their persistent presence and linking fluctuation timescales to observable power spectral features, thus providing insights into the MRI mechanism.

Contribution

It demonstrates the widespread presence of propagating fluctuations in non-linear disc models and establishes a quantitative relationship between fluctuation timescales and PSD break frequencies.

Findings

Propagating fluctuations are present across all simulated models.

The fluctuation timescale strongly influences the PSD break frequency.

A fitting formula relates the break frequency to disc parameters and fluctuation timescale.

Abstract

Across a large range of scales, accreting sources show remarkably similar patterns of variability, most notably the log-normality of the luminosity distribution and the linear root-mean square (rms)-flux relationship. These results are often explained using the theory of propagating fluctuations in which fluctuations in the viscosity create perturbations in the accretion rate at all radii, propagate inwards and combine multiplicatively. While this idea has been extensively studied analytically in a linear regime, there has been relatively little numerical work investigating the non-linear behaviour. In this paper, we present a suite of stochastically driven 1-d $\alpha$-disc simulations, exploring the behaviour of these discs. We find that the eponymous propagating fluctuations are present in all simulations across a wide range of model parameters, in contradiction to previous work. Of the model parameters, we find by far the most important to be the timescale on which the viscosity fluctuations occur. Physically, this timescale will depend on the underlying physical mechanism, thought to be the magnetorotational instability (MRI). We find a close relationship between this fluctuation timescale and the break frequency in the power spectral density (PSD) of the luminosity, a fact which could allow observational probes of the behaviour of the MRI dynamo. We report a fitting formula for the break frequency as a function of the fluctuation timescale, the disc thickness and the mass of the central object.