Testing the Propagating Fluctuations Model with a Long, Global Accretion Disk Simulation

TL;DR

This study uses a high-resolution 3D MHD simulation of a black hole accretion disk to investigate propagating fluctuations, successfully reproducing observed variability features like log-normal flux distributions and RMS-flux relations.

Contribution

First analysis linking propagating fluctuations in accretion disks to MHD simulations, demonstrating how low-frequency dynamo action drives observed variability characteristics.

Findings

Log-normal flux distributions observed

RMS-flux relations are linear

Radial coherence and inter-band lags are reproduced

Abstract

The broad-band variability of many accreting systems displays characteristic structure; log-normal flux distributions, RMS-flux relations, and long inter-band lags. These characteristics are usually interpreted as inward propagating fluctuations in an accretion disk driven by stochasticity of the angular momentum transport mechanism. We present the first analysis of propagating fluctuations in a long-duration, high-resolution, global three-dimensional magnetohydrodynamic (MHD) simulation of a geometrically-thin ($h/r\approx0.1$) accretion disk around a black hole. While the dynamical-timescale turbulent fluctuations in the Maxwell stresses are too rapid to drive radially-coherent fluctuations in the accretion rate, we find that the low-frequency quasi-periodic dynamo action introduces low-frequency fluctuations in the Maxwell stresses which then drive the propagating fluctuations. Examining both the mass accretion rate and emission proxies, we recover log-normality, linear RMS-flux relations, and radial coherence that would produce inter-band lags. Hence, we successful relate and connect the phenomenology of propagating fluctuations to modern MHD accretion disk theory.