A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*

Abstract

The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the existence of a compact emitting component within several Schwarzschild radii, $r_S$, of the nucleus at the Galactic Center. This is interesting in view of the predicted circularized flow within $\sim 5-10r_S$, based on detailed multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this unusual object. In this paper, we examine the physics of magnetic field generation by a Keplerian dynamo subject to the conditions pertaining to Sgr A*, and show that the sub-mm bump can be produced by thermal synchrotron emission in this inner region. This spectral feature may therefore be taken as indirect evidence for the existence of this circularization. In addition, the self-Comptonization of the sub-mm bump appears to produce an X-ray flux exceeding that due to bremsstrahlung from this region, which may account for the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However, the required accretion rate in the Keplerian flow is orders of magnitude smaller than that predicted by the Bondi-Hoyle simulations. We speculate that rapid evaporation, in the form of a wind, may ensue from the heating associated with turbulent mixing of gas elements with large eccentricity as they settle down into a more or less circular (i.e., low eccentricity) trajectory. The spectrum of Sgr A* longward of $\sim 1-2$ mm may be generated outside of the Keplerian flow, where the gas is making a transition from a quasi-spherical infall into a circularized pattern.

40 pages, 9 figures

References in corpus (3)

• Global Magnetohydrodynamical Simulations of Accretion Tori
• The Effect of Resistivity on the Nonlinear Stage of the Magnetorotational Instability in Accretion Disks
• The Role of Magnetic Field Dissipation in the Black Hole Candidate Sgr A*

Cited by in corpus (44)

• Hot Accretion Flows Around Black Holes
• Chandra X-ray Spectroscopic Imaging of Sgr A* and the Central Parsec of the Galaxy
• Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A*
• The Dynamical Structure of Nonradiative Black Hole Accretion Flows
• An Unambiguous Detection of Faraday Rotation in Sagittarius A*
• A Jet-ADAF model for Sgr A*
• The Nature of the 10 Kilosecond X-ray Flare in Sgr A*
• An X-ray, IR, and Submillimeter Flare of Sagittarius A*
• A New X-Ray Flare from the Galactic Nucleus Detected with the XMM-Newton Photon Imaging Cameras
• Polarimetric Imaging of the Massive Black Hole at the Galactic Center
• An Accretion-Induced X-ray Flare in Sgr A*
• Variations in the spectral slope of Sgr A* during a NIR flare
• Electron Acceleration around the Supermassive Black Hole at the Galactic Center
• The Rotation Measure and 3.5mm Polarization of Sgr A*
• Stochastic Electron Acceleration During the NIR and X-ray Flares in Sagittarius A*
• Stochastic Acceleration in the Galactic Center HESS Source
• Spin-Induced Disk Precession in the Supermassive Black Hole at the Galactic Center
• MHD Simulations of Accretion onto Sgr A*: Quiescent Fluctuations, Outbursts, and Quasi-Periodicity
• Direct Feeding of the Black Hole at the Galactic Center with Radial Gas Streams from Close-In Stellar Winds
• New Constraints on the Nature of Radio Emission in Sagittarius A*
• The Circular Polarization of Sagittarius A* at Submillimeter Wavelengths
• A Testable the Stochastic Acceleration Model for Flares in Sagittarius A*
• Testing RIAF model for Sgr A* using the size measurements
• The Atacama Cosmology Telescope: Microwave Intensity and Polarization Maps of the Galactic Center
• Testing the Accretion Flow with Plasma Wave Heating Mechanism for Sagittarius a* by the 1.3MM Vlbi Measurements
• Towards Self-Consistent Modelling of the Sgr A* Accretion Flow: Linking Theory and Observation
• Polarized Emission of Sagittarius A*
• On the radio dichotomy of active galactic nuclei
• The QUaD Galactic Plane Survey II: A Compact Source Catalog
• Linearly and Circularly Polarized Emission in Sagittarius A*
• Low angular momentum flow model for Sgr A*
• Accretion models of Sgr A*
• The Nature of Linearly Polarized Millimeter and Sub-millimeter Emission in Sagittarius A*
• Does the circularization radius exist or not for low angular momentum accretion?
• Modulated X-ray Emissivity near the Stress Edge in Sgr A*
• Spin-Induced Disk Precession in Sagittarius A*
• Constraints on jet-driven disk accretion in Sagittarius A*
• Correlation between Flux and Spectral Index during Flares in Sagittarius A*
• Low angular momentum flow model of Sgr A* activity
• Effect of matter geometry on low angular momentum black hole accretion in the Kerr metric
• Influence of flow thickness on general relativistic low angular momentum accretion around spinning black holes
• A Relativistic Disk in Sagittarius A*
• A New X-Ray Flare from the Galactic Nucleus Detected with the XMM-Newton
• Line Emission from Cooling Accretion Flows in the Nucleus of M31