We have placed an upper limit on the magnitude of the rotation measure of 7 × 10^5 rad m-2 for the putative Faraday screen (magnetized plasma) in front of Sgr A*, the radio source associated with the black hole in the Galactic Center. There is evidence that the actual rotation measure is about -5 ×105 rad m-2. With a simple model of equipartition of energy and reasonable inner radius for the screen, the accretion rate is estimated to be less than 10-6M⊙yr-1. In addition, we have detected, for the first time, intra-day variability in the polarization of Sgr A*, which may be due to either intrinsic variations in Sgr A* or variations in the composition of the Faraday screen.
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Moran, J., Marrone, D., Zhao, JH., Rao, R. (2007). The Polarization Properties of Sgr A∗ at Submillimeter Wavelengths. In: Aschenbach, B., Burwitz, V., Hasinger, G., Leibundgut, B. (eds) Relativistic Astrophysics Legacy and Cosmology – Einstein’s. ESO Astrophysics Symposia. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-74713-0_37
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