Accretion disc dynamo activity in local simulations spanning weak-to-strong net vertical magnetic flux regimes

TL;DR

This study investigates how the strength of net vertical magnetic flux influences dynamo activity and disc structure in accretion discs around black holes through 3D MHD simulations, revealing key dependencies and analytic model matches.

Contribution

It provides a comprehensive analysis of dynamo activity across a wide range of magnetic flux regimes using stratified shearing box simulations, extending understanding of magnetic field evolution in accretion discs.

Findings

Dynamo activity persists up to $eta_0^{ m mid} = 10^2$

Effective $eta^{ m mid}$ matches Begelman et al. (2015) model

Strong fields lead to inhomogeneous, fluctuation-rich disc structure

Abstract

Strongly magnetized accretion discs around black holes have attractive features that may explain enigmatic aspects of X-ray binary behaviour. The structure and evolution of these discs are governed by a dynamo-like mechanism, which channels part of the accretion power liberated by the magnetorotational instability (MRI) into an ordered toroidal magnetic field. To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations. The strength of the self-sustained toroidal magnetic field depends on the net vertical magnetic flux, which we vary across almost the entire range over which the MRI is linearly unstable. We quantify disc structure and dynamo properties as a function of the initial ratio of mid-plane gas pressure to vertical magnetic field pressure, $\beta_0^{\rm mid} = p_{\rm gas} / p_B$. For $10^5 \geq \beta_0^{\rm mid} \geq 10$ the effective $\alpha$-viscosity parameter scales as a power-law. Dynamo activity persists up to and including $\beta_0^{\rm mid} = 10^2$, at which point the entire vertical column of the disc is magnetic pressure-dominated. Still stronger fields result in a highly inhomogeneous disc structure, with large density fluctuations. We show that the turbulent steady state $\beta^{\rm mid}$ in our simulations is well-matched by the analytic model of Begelman et al. (2015) describing the creation and buoyant escape of toroidal field, while the vertical structure of the disc can be broadly reproduced using this model. Finally, we discuss the implications of our results for observed properties of X-ray binaries.