A Jet-ADAF model for Sgr A*
Feng Yuan1, Sera Markoff2,
1 Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, D-53121 Bonn, Germany
2 Massachusetts Institute of Technology, Center for Space
Research, 77 Massachusetts Avenue,
Cambridge, MA 02139, USA
E-mail contact: email@example.com
See A&A, 383, 854 (2002)
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 ~2 x 10^11 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
can 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.