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Title: LOW-FREQUENCY OSCILLATIONS IN GLOBAL SIMULATIONS OF BLACK HOLE ACCRETION

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 have previously been seen in local shearing box simulations, but we discuss their evolution over 1500 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 10-20 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, narrowband 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 LFQPOmore » 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.« less

Authors:
 [1]; ; ;  [2]
  1. JILA, University of Colorado, 440 UCB, Boulder, CO 80309 (United States)
  2. Department of Astronomy and Maryland Astronomy Center for Theory and Computation, University of Maryland, College Park, MD 20742 (United States)
Publication Date:
OSTI Identifier:
21578264
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 736; Journal Issue: 2; Other Information: DOI: 10.1088/0004-637X/736/2/107; Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; BLACK HOLES; EMISSION; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; OSCILLATIONS; SIMULATION; X RADIATION; ELECTROMAGNETIC RADIATION; FLUID MECHANICS; HYDRODYNAMICS; IONIZING RADIATIONS; MECHANICS; RADIATIONS

Citation Formats

O'Neill, Sean M., Reynolds, Christopher S., Coleman Miller, M., and Sorathia, Kareem A. LOW-FREQUENCY OSCILLATIONS IN GLOBAL SIMULATIONS OF BLACK HOLE ACCRETION. United States: N. p., 2011. Web. doi:10.1088/0004-637X/736/2/107.
O'Neill, Sean M., Reynolds, Christopher S., Coleman Miller, M., & Sorathia, Kareem A. LOW-FREQUENCY OSCILLATIONS IN GLOBAL SIMULATIONS OF BLACK HOLE ACCRETION. United States. doi:10.1088/0004-637X/736/2/107.
O'Neill, Sean M., Reynolds, Christopher S., Coleman Miller, M., and Sorathia, Kareem A. Mon . "LOW-FREQUENCY OSCILLATIONS IN GLOBAL SIMULATIONS OF BLACK HOLE ACCRETION". United States. doi:10.1088/0004-637X/736/2/107.
@article{osti_21578264,
title = {LOW-FREQUENCY OSCILLATIONS IN GLOBAL SIMULATIONS OF BLACK HOLE ACCRETION},
author = {O'Neill, Sean M. and Reynolds, Christopher S. and Coleman Miller, M. and Sorathia, Kareem A.},
abstractNote = {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 have previously been seen in local shearing box simulations, but we discuss their evolution over 1500 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 10-20 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, narrowband 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.},
doi = {10.1088/0004-637X/736/2/107},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 736,
place = {United States},
year = {2011},
month = {8}
}