Low-Frequency Oscillations in Global Simulations of Black Hole Accretion

TL;DR

This study identifies large-scale, low-frequency dynamo cycles in global MHD simulations of black hole accretion disks, revealing potential links to observed low-frequency QPOs but highlighting the need for further research.

Contribution

First demonstration of global dynamo cycles in long-duration black hole accretion simulations, connecting simulation results to observed low-frequency variability.

Findings

Dynamo cycles occur at frequencies 10-20 times lower than orbital frequency.

Cycles manifest as narrow-band oscillations in magnetic fields.

Simulated cycle frequencies are consistent with LFQPOs in black hole systems.

Abstract

We have identified the presence of large-scale, low-frequency dynamo cycles in a long-duration, global, magnetohydrodynamic (MHD) simulation of black hole accretion. Such cycles had been seen previously in local shearing box simulations, but we discuss their evolution over 1,500 inner disk orbits of a global pi/4 disk wedge spanning two orders of magnitude in radius and seven scale heights in elevation above/below the disk midplane. The observed cycles manifest themselves as oscillations in azimuthal magnetic field occupying a region that extends into a low-density corona several scale heights above the disk. The cycle frequencies are ten to twenty times lower than the local orbital frequency, making them potentially interesting sources of low-frequency variability when scaled to real astrophysical systems. Furthermore, power spectra derived from the full time series reveal that the cycles manifest themselves at discrete, narrow-band frequencies that often share power across broad radial ranges. We explore possible connections between these simulated cycles and observed low-frequency quasi-periodic oscillations (LFQPOs) in galactic black hole binary systems, finding that dynamo cycles have the appropriate frequencies and are located in a spatial region associated with X-ray emission in real systems. Derived observational proxies, however, fail to feature peaks with RMS amplitudes comparable to LFQPO observations, suggesting that further theoretical work and more sophisticated simulations will be required to form a complete theory of dynamo-driven LFQPOs. Nonetheless, this work clearly illustrates that global MHD dynamos exhibit quasi-periodic behavior on timescales much longer than those derived from test particle considerations.