Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-07-08
Astron.Nachr.324S1:475-481,2003
Astronomy and Astrophysics
Astrophysics
12 pages including 4 figures
Scientific paper
10.1002/asna.200385045
The detection of a mm/Sub-mm ``bump'' in Sgr A*'s radio spectrum suggests that at least a portion of its overall emission is produced within a compact accretion torus. This inference is strengthened by observations of strong linear polarization (at the 10 percent level) within this bump. No linear polarization has been detected yet at other wavelengths. Given that radiation from this source is produced on progressively smaller spatial scales with increasing frequency, the mm/Sub-mm bump apparently arises within a mere handful of Schwarzschild radii of the black hole. We have found that a small (10-Schwarzschild-radii) magnetized accretion torus can not only account for the spectral bump via thermal synchrotron processes, but that it can also reproduce the corresponding polarimetric results. In addition, the quiescent X-ray emission appears to be associated with synchrotron self-Comptonization, while X-ray flares detected from Sgr A* may be induced by a sudden enhancement of accretion through this torus. This picture predicts correlations among the mm, IR, and X-ray flux densities, that appear to be consistent with recent multi-wavelength observations. Further evidence for such a torus in Sgr A* is provided by its radio variability. Recent monitoring of Sgr A* at cm and mm wavelengths suggests for a spectral break at 3 mm during cm/Sub-mm flares. Radio emission from Sgr A* is intriguing since it is circularly polarized though no linear polarization was ever detected. We first proved that the emission must be due to nonthermal process should the flow be bounded. It is then suggested that the nonthermal electrons are energized very close to the black hole.
Liu Siming
Melia Fulvio
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