Linear and Circular Polarization in M81

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 m_{c}=0.54 +/- 0.06 +/- 0.07 % at 8.4 GHz and m_{c}=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 AGN. 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 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.

We analyzed archival Very Large Array (VLA) data and new VLA observations to investigate the long term behavior of the circular polarization of M81*. We also used the Berkeley-Illinois-Maryland Association (BIMA) array to observe M81* at 86 and 230 GHz. M81* is unpolarized in the linear sense at a frequency as high as 86 GHz and shows variable circular polarization at a frequency as high as 15 GHz. The spectrum of the fractional circular polarization is inverted in most of our observations. The sign of circular polarization is constant over frequency and time. The absence of linear polarization sets a lower limit to the accretion rate of $10^{-7} M_\odot y^{-1}$. The polarization properties are strikingly similar to the properties of Sgr A*, the central radio source in the Milky Way. This supports the hypothesis that M81* is a scaled up version of Sgr A*. On the other hand, the broad band total intensity spectrum declines towards milimeter wavelengths which differs from previous observations of M81* and also from Sgr A*.

Published in:

Brunthaler A., Bower G.C., Falcke H., Mellon R.R. 2001, ApJ 560, L123
Brunthaler A., Bower G.C., Falcke H., 2006, A&A 451, 845

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Circular polarization (contours) and total intensity (grey sale) VLA-map of M81* at 8.4 GHz.

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Total Intensity and fractional circular polarization lightcurves of M81* at 8.4 GHz.

Last modified: 13-July-2012