Radio linear and circular polarization from M 81^* 

¹ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands e-mail: brunthal@mpifr-bonn.mpg.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA
⁴ ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁵ Department of Astrophysics, Radboud Universiteit Nijmegen, Postbus 9010, 6500 GL Nijmegen, The Netherlands

Received: 30 December 2005
Accepted: 19 January 2006

Abstract

We present results from archival and new Very Large Array (VLA) data observations to investigate the long term behavior of the circular polarization of M 81*, the nuclear radio source in the nearby galaxy M 81. We also used the Berkeley-Illinois-Maryland Association (BIMA) array to observe M 81* at 86 and 230 GHz. M 81* 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 . 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 M 81* 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 M 81* and also from Sgr A*.