Abstract
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 estimate a value of mc = 0.54% ± 0.06% ± 0.07% at 8.4 GHz and mc = 0.27% ± 0.06% ± 0.07% at 4.8 GHz for the fractional circular polarization. The errors are separated into statistical and systematic terms. The spectrum of the circular polarization is possibly inverted, which would be unusual for active galactic nuclei. We also detected no linear polarization in M81* at a level of 0.1%, implying that the source has a very high circular-to-linear polarization ratio as found so far only in Sagittarius 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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