A magnetic dynamo origin for the submillimeter excess in sagittarius A

Fulvio Melia, Siming Liu, Robert Coker

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40 Scopus citations


The submillimeter bump observed in the spectrum of Sgr A * appears to indicate the existence of a compact emitting component within several Schwarzschild radii, rs, of the nucleus at the Galactic center. This is interesting in view of the predicted circularized flow within ∼5-10rs, based on detailed multidimensional 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 submillimeter 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 submillimeter 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 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 ∼1-2 mm may be generated outside of the Keplerian flow, where the gas is making a transition from a quasi-spherical infall to a circularized pattern.

Original languageEnglish (US)
Pages (from-to)146-157
Number of pages12
JournalAstrophysical Journal
Issue number1 PART 1
StatePublished - May 20 2001


  • Accretion, accretion disks
  • Black hole physics
  • Galaxy: Center
  • Hydrodynamics
  • MHD
  • Magnetic fields

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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