Subject: GCFLASH - Vol. 5, No. 5 (Jun 13, 1997)

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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@astro.umd.edu                    http://www.astro.umd.edu/~gcnews
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Vol. 5, No. 5                                              Jun 13, 1997


Recently submitted papers:
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Email    : rohan@kailash.harvard.edu
Title    : Advection-Dominated Accretion Model of Sagittarius A^*:   
           Evidence for a Black Hole at the Galactic Center
Author(s): Ramesh Narayan, Rohan Mahadevan, Jonathan E. Grindlay,    
           Robert G. Popham, Charles Gammie
Institute: (1) Harvard-Smithsonian Center for Astrophysics, 60 Garden 
           St.,   Cambridge, MA 02138
Paper    : submitted to ApJ
EPrint   : astro-ph/9706112

Abstract:
Sgr A^* at the Galactic Center is a puzzling source.  It has a mass
M=(2.5+/-0.4)*10^6 M_o which makes it an excellent black hole
candidate.  Observations of stellar winds and other gas flows in its
vicinity suggest a mass accretion rate \dot M\gsim few*10^-6\msun
yr^-1.  However, such an accretion rate would imply a luminosity >
10^40 erg s^-1 if the radiative efficiency is the usual 10%, whereas
observations indicate a bolometric luminosity <10^37 erg s^-1.  The
spectrum of Sgr A^* is unusual, with emission extending over many
decades of wavelength.  We present a model of Sgr A^* which is based
on a two-temperature optically-thin advection-dominated accretion
flow.  The model is consistent with the estimated M and \dot M, and it
fits the observed fluxes in the radio and X-ray bands as well as
measured upper limits in the sub-millimeter and infrared bands.  The
model explains the very low luminosity of Sgr A^* by invoking
advection.  Most of the viscously dissipated energy is carried into
the central star by the accreting gas, and therefore the radiative
efficiency is extremely low, ~5*10^-6.  A critical element of the
model is the presence of an event horizon at the center.  The success
of the model may thus be viewed as confirmation that Sgr A^* is a
black hole.

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Angela Cotera                                 Heino Falcke
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