A Jet-ADAF model for Sgr A^*

Feng Yuan (1,2), Sera Markoff (1), Heino Falcke (1)


(1) Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
(2) Also at CAS-PKU Joint Beijing Astrophysical Centre, Beijing 100871, China; and Astronomical and Astrophysical Center of East China, Nanjing University, Nanjing 210093, China.

Paper: A&A, in press

EPrint Server: astro-ph/0112464


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 2 * 1011K, 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.


Preprints available from the authors at hfalcke@sun184.mpifr-bonn.mpg.de , or the raw TeX (no figures) if you click here.

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