Polarization in Sagittarius A*
  We summarize the current state of polarization observations of Sagittarius A*, the compact radio source and supermassive black hole candidate in the Galactic Center. These observations are providing new tools for understanding accretion disks, jets and their environments. Linear polarization observations have shown that Sgr A* is unpolarized at frequencies as high as 86 GHz. However, recent single-dish observations indicate that Sgr A* may have strong linear polarization at frequencies higher than 150 GHz. Circular polarization, which was recently discovered in Sgr A*, is detectable to frequencies as high as 43 GHz. It is strongly variable on a 10-day timescale, with the degree of variability increasing with frequency. At low frequencies, fractional circular polarization is stable over nearly 20 years. We discuss some of the possible models for the origin of the circular polarization. 
Geoffrey C. Bower