Finally, as a relatively recent development, the polarization properties of Sgr A^* are now relatively well established. For a long time it was generally thought that, in marked contrast to more luminous AGNs, Sgr A^* was unpolarized. This is true for linear polarization in the GHz radio regime. Bower et al (1999a) found an upper limit of ≤ 0.1% to the linear polarization at cm waves; however, they also found plenty of circular polarization.

The results for linear polarization at low frequencies were obtained with a rarely used technique in radio astronomy, called spectro-polarimetry. This allows one to look for polarization in small frequency bands. Usually in continuum observations one averages the polarization of the radiation over the available bandwidth δν. However, since Sgr A^* may be embedded in a dense plasma, Faraday rotation in the accretion flow or a foreground Galactic screen could lead to a rotation of the linear polarization vector even within the small bandwidth. Faraday rotation is produced when radio waves pass through an ionized and magnetized medium. Since left and right circularly polarized waves have different refractive indices for a given magnetic field orientation, a wavelength-dependent delay is induced that rotates the position angle φ of the linear polarization vector by an amount

 Δφ = RMλ².     (1.1)

The parameter RM is called the rotation measure and can be determined by measuring the position angle of the linear polarization vector φ at different wavelengths. For a given frequency bandwidth δν, significant de-polarization is obtained if Δφ ∼ 1 rad. Hence, for a typical VLA bandwidth of Δν = 50 MHz at 4.8 GHz a rotation measure of RM = 10⁴ rad m⁻² of any foreground material would destroy any intrinsic polarization signal. Such a value for RM is large compared to what is seen in other AGNs but cannot be excluded in the Galactic Center. By Fourier-transforming spectro-polarimetric data (to look for periodic signals due to the fast rotation of the polarization vector as a function of frequency), Bower et al (1999a) were able to set the 0.1% limit and also exclude rotation measures below RM < 10⁷ rad m⁻² for a homogeneous foreground screen as the cause for the low polarization.

Later, Aitken et al (2000) made linear polarization observations with a single dish sub-millimeter wave telescope and found ∼10% linear polarization above 150 GHz. This was confirmed with an interferometer by Bower et al (2002b) and they also found evidence for a large rotation measure < 10⁶ rad m⁻² plus some evidence for intrinsic depolarization towards lower frequencies. This may be the first direct evidence for a hot accretion flow around Sgr A^*. At the moment of writing this is a strongly developing field which promises many new insights in the future. For example, one can use the measured RM to limit the accreting material engulfing Sgr A^* (Agol 2000) (see also p 335).

As a big surprise, Bower et al (1999b) also found strong circular polarization at the 0.3-1% level. This is unusual because typical AGN polarization is more linear than circular and it can be used to constrain the electron content and distribution in Sgr A^*. Interestingly, the circular polarization itself turned out to be variable. At higher frequencies the variability as well as the fractional polarization increases (Bower et al 2002a). Summarizes the currently known polarization properties of the Sgr A^* spectrum.

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