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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@mpifr-bonn.mpg.de             http://www.mpifr-bonn.mpg.de/gcnews
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Vol. 10, No. 18                                              Sep 28, 1999


Recently submitted papers:
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Email    : rfc@gc.physics.arizona.edu
Title    : The Role of Magnetic Field Dissipation in the Black Hole 
           Candidate Sgr A*
Author(s): Robert F. Coker(1,3), Fulvio    Melia(1,2,4)
Institute: (1) Physics Department, The University of Arizona, Tucson, 
           AZ 85721   (2) Steward Observatory, The University of 
           Arizona, Tucson, AZ    85721   (3) NASA GSRP Fellow.   
           (4) Sir Thomas Lyle Fellow.
Paper    : ApJ, submitted
EPrint   : astro-ph/9909411

Abstract:
The compact, nonthermal radio source Sgr A* at the Galactic Center
appears to be coincident with a ~ 2.6* 10^6 M_o point-like object.
Its energy source may be the release of gravitational energy as gas
from the interstellar medium descends into its deep potential well.
However, simple attempts at calculating the radiative spectrum and
flux based on this picture have come tantalizingly close to the
observations, yet have had difficulty in accounting for the unusually
low efficiency in this source.  Regardless of whether the radiating
particles in the accretion flow are thermal or nonthermal, there now
appear to be two principal reasons for this low conversion rate of
dissipated energy into radiation: (1) the plasma separates into two
temperatures, with the protons attaining a significantly higher
temperature than that of the radiating electrons, and (2) the magnetic
field B is sub-equipartition, which reduces the magnetic
bremsstrahlung emissivity, and therefore the overall power of Sgr A*.
In this paper, we investigate the latter with a considerable
improvement over what has been attempted before. In particular, rather
than calculating B based on some presumed model (e.g., equipartition
with the thermal energy of the gas), we instead infer its distribution
with radius empirically with the requirement that the resulting
spectrum matches the observations.  Our assumed ansatz for B(r) is
motivated in part by earlier calculations of the expected magnetic
dissipation rate due to reconnection in a compressed flow.  We find
reasonable agreement with the observed spectrum of Sgr A* as long as
its distribution consists of 3 primary components: an outer
equipartition field, a roughly constant field at intermediate radii (~
10^3 Schwarzschild radii), and an inner dynamo (more or less within
the last stable orbit for a non-rotating black hole) which increases B
to about 100 Gauss.  The latter component accounts very well for the
observed sub-millimiter hump in this source.

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