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The Influence of Accretion Disk Thickness on the Large-scale Magnetic Dynamo

Accepted version
Peer-reviewed

Type

Article

Change log

Authors

Hogg, JD 
Reynolds, CS 

Abstract

The evolution of the magnetic field from the large-scale dynamo is considered a central feature of the accretion disk around a black hole. The resulting low-frequency oscillations introduced from the growth and decay of the field strength, along with the change in field orientation, play an integral role in the accretion disk behavior. Despite the importance of this process and how commonly it is invoked to explain variable features, it still remains poorly understood. We present a study of the dynamo using a suite of four global, high-resolution, MHD accretion disk simulations. We systematically vary the scale height ratio and find the large-scale dynamo fails to organize above a scale height ratio of h/r≳0.2. Using spacetime diagrams of the azimuthal magnetic field, we show the large-scale dynamo is well-ordered in the thinner accretion disk models, but fails to develop the characteristic "butterfly" pattern when the scale height ratio is increased, a feature which is also reflected in the power spectra. Additionally, we calculate the dynamo α-parameter and generate synthetic light curves. Using an emission proxy, we find the disks have markedly different characters as stochastic photometric fluctuations have a larger amplitude when the dynamo is unordered.

Description

Keywords

accretion, accretion disks, black hole physics, dynamo, magnetohydrodynamics (MHD)

Journal Title

Astrophysical Journal

Conference Name

Journal ISSN

0004-637X
1538-4357

Volume Title

861

Publisher

American Astronomical Society
Sponsorship
National Aeronautics and Space Administration (NNX16AP88H)