Circular Polarization and the Radiation Efficiency of Sgr A*

Thomas Beckert1

1 Max-Planck-Institut für Radioastromonie, Auf dem Hügel 69, 53121 Bonn, Germany

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See: A&A, 388, 1106 (2002)

The radio and X-ray source Sgr A* in the Galactic Center is discussed in the light of circular and linear polarization measurements at radio wavelengths. The flat to inverted radio spectrum is interpreted as a signpost for an outflow driven by accretion onto a central super-massive black hole. The lack of linear polarization at GHz frequencies may either be due to depolarization in the accretion flow or due to a large Faraday rotation depth in the outflow itself. We discuss the generation of circular polarization together with the suppression of linear polarization by intrinsic Faraday rotation and conversion in the jet. These processes require a large population of `invisible' low-energy electrons in an electron/proton-plasma. Together with the high linear polarization at sub-mm wavelengths a specific magnetic configuration in the jet is implied. As a consequence the kinetic energy of the jet is much larger than expected from a standard jet model and the radio and X-ray emission traces only a small fraction of the kinetic luminosity of Sgr A*. The mass accretion rate required to power the jet is consistent with accretion from the diffuse gas around Sgr A* and suggests a low radiation efficiency for both the jet and accretion flow. The observed radio-polarization and X-ray flux leads to a well-constrained model for the outflowing matter from Sgr A*.