Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
F. Yuan, S. Markoff, and H. Falcke:
A Jet-ADAF model for Sgr A*
Astronomy & Astrophysics 383, 854-863
(2002)
Abstract.
The recent Chandra observation of the radio source at the center
of our Galaxy, Sgr A*, puts new constraints on its theoretical
models. The spectrum is very soft, and the source is rapidly variable.
We consider different models to explain the observations. We find that
the features of the x-ray spectrum can be marginally explained with an
advection-dominated accretion flow (ADAF) model while it does not well
fit the radio spectrum. An ADAF with strong winds (ADIOS) model is not
favored if we assume that the wind does not radiate. Alternatively, we
propose a coupled jet plus accretion disk model to explain the
observations for Sgr A*. The accretion flow is described as an ADAF
fed by Bondi-Hoyle accretion of hot plasma in the Galactic Center
region. A small fraction of
the accretion flow is ejected near the black hole, forming a jet after
passing through a shock. As a result, the electron temperature
increases
to ~2x10^11 K, which is about 10 times higher than
the highest temperature attained in the ADAF. The model is
self-consistent since the main jet parameters are determined by the
underlying accretion disk at the inner edge. The emergent spectrum of
Sgr A* is the sum of the emission from jet and underlying ADAF. The
very strong Comptonization of synchrotron emission from the jet
dominates the bremsstrahlung from the ADAF, therefore, a very short
variability timescale is expected and the predicted X-ray slope and the
radio spectrum is in very good agreement with the observations.
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