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Title:
MHD Simulations of Accretion onto Sgr A*: Quiescent Fluctuations, Outbursts, and Quasiperiodicity
Authors:
Chan, Chi-kwan; Liu, Siming; Fryer, Christopher L.; Psaltis, Dimitrios; Özel, Feryal; Rockefeller, Gabriel; Melia, Fulvio
Affiliation:
AA(Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA ; Department of Physics, the University of Arizona, Tucson, AZ 85721, USA ), AB(Department of Physics and Astronomy, University of Glasgow, Kelvin Bldg Rm 620, Glasgow G12 8QQ, UK ), AC(Department of Physics, the University of Arizona, Tucson, AZ 85721, USA ; Computational Computer Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA ), AD(Department of Physics, the University of Arizona, Tucson, AZ 85721, USA ; Steward Observatory, the University of Arizona, Tucson, AZ 85721, USA ), AE(Department of Physics, the University of Arizona, Tucson, AZ 85721, USA ; Steward Observatory, the University of Arizona, Tucson, AZ 85721, USA ), AF(Computational Computer Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA ), AG(Department of Physics, the University of Arizona, Tucson, AZ 85721, USA ; Steward Observatory, the University of Arizona, Tucson, AZ 85721, USA ; Sir Thomas Lyle Fellow and Miegunyah Fellow.)
Publication:
The Astrophysical Journal, Volume 701, Issue 1, pp. 521-534 (2009). (ApJ Homepage)
Publication Date:
08/2009
Origin:
IOP
ApJ Keywords:
accretion, accretion disks, black hole physics, Galaxy: center, instabilities, MHD, relativity
DOI:
10.1088/0004-637X/701/1/521
Bibliographic Code:
2009ApJ...701..521C

Abstract

High-resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations (QPOs), making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudospectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic (MHD) model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme and calibrating the parameters by observations, we show that the MHD turbulence in the environment of Sgr A* can by itself only produce factor two fluctuations in luminosity. These fluctuations cannot explain the magnitude of flares observed in this system. However, we also demonstrate that external forcing of the accretion disk, which may be generated by the "clumpy material" raining down onto the disk from the large-scale flow, do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and by ground-based facilities in the near infrared. Strong, but short-term QPOs emerge naturally in the simulated light curves. We attribute these to nonaxisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state.
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