1996A&A...310L...5R

Radio continuum observations at short mm-wavelengths (λλ3.4, 2.2 and 1.2mm) of the nucleus of NGC3031 (M81) allow a final determination of the radio spectrum from the meter to the short mm-range. The spectrum is found to be interpreted up to 100GHz and is well fitted by a S_ν~ν^1/3exp(-ν/ν_c) law with a break frequency ν_c=200HGHz. The radio spectrum is discussed in terms of two homogeneous and inhomogeneous synchrotron self-absorption models, however, each of these models requires quite unlikely physical parameters in order to be self-consistent. As an alternative scenario, we present a model of optically thin synchrotron emission of a quasi-monoenergetic distribution of relativistic electrons which, in the ideal case, shows exactly a S_ν~ν^1/3exp(-ν/ν_c) behaviour. We find a striking similarity of the M81 spectrum with that of the radio core in the Milky Way (SgrA*) which may lead to the conclusion that the same underlying physical mechanism is responsible for the observed spectra and that the core of M81 is a by a factor of 10000 upscaled version of SgrA*.