Yuan, Markoff, Falcke 2001: Jet-ADAF Model for Sgr A*
A Jet-ADAF model for Sgr A*
Feng Yuan, Sera Markoff, & Heino Falcke
Max-Planck-Institut für Radioastronomie, Auf den Hügel 69, D-53121 Bonn, Germany
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 these results. We find that
these features are hard to explain with an advection-dominated
accretion flow (ADAF) model alone because the predicted spectrum is
too hard and the variability timescale is too long. This remains true
if one considers the possibility of winds from an ADAF - assuming
viscous dissipation preferentially heats ions rather than electrons
and 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*1011 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 is in
very good agreement with the observations.
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