Brunthaler, Bower, Falcke, Melon 2001: Circular Polarization in M81

Detection of Circular Polarization in M81

Andreas Brunthaler 1,2 Geoffrey C. Bower3, Heino Falcke1, Rick R. Melon4,

1Harvard-Smithsonian Center for Astrophysics, 60 Garden Street MS 42, Cambridge, MA 02138 (
2Max-Planck-Institut für Radioastronomie, Auf den Hüugel 69, D-53121 Bonn, Germany (
3UC Berkeley, RAL, 601 Campbell Hall, Berkeley CA 94720
4Department of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802

The Astrophysical Journal Letters, Vol. 560, L123 (2001)


We report the detection of circular polarization in the compact radio jet of the nearby spiral galaxy M81 (M81*). The observations were made with the Very Large Array at 4.8 and 8.4 GHz and circular polarization was detected at both frequencies. We estimated a value of m_c=0.54% at 8.4 GHz and m_c=0.26% at 4.8 GHz for the fractional circular polarization. The spectrum of the circular polarization is inverted which is unusual for AGN. We also detected no linear polarization in M81* to a level of 0.01% implying that the source has a very high circular-to-linear polarization ratio found so far only in Sgr A*, the central radio source in our Galaxy. This further supports the idea that M81* is a scaled-up version of Sgr A* and suggests that the polarization properties are intrinsic to the two sources and are not caused by a foreground screen in the Galaxy.

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Questions: Heino Falcke,