Low-Frequency Oscillations in Global Simulations of Black Hole Accretion
  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.
