A Jet-ADAF model for Sgr A*

Feng Yuan, Sera Markoff, & Heino Falcke

Astronomy & Astrophysics (2002), Vol. 383, p. 854

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 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.

Paper: Available in PostScript and (AA)LaTex. Please send an email request to hfalcke@mpifr-bonn.mpg.de for a preprint.

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