The millimetre variability of M81*-: Multi-epoch dual frequency mm-observations of the nucleus of M81

Aims. There are still many open questions as to the physical mechanisms at work in Low Luminosity AGN that accrete in the extreme sub-Eddington regime. Simultaneous multi-wavelength studies have been very successful in constraining the properties of Sgr A*, the extremely sub-Eddington black hole at the centre of our Milky Way. M81*, the nucleus of the nearby spiral galaxy M81, is an ideal source to extend the insights obtained on Sgr A* toward higher luminosity AGN. Here we present observations at 3 and 1mm that were obtained within the framework of a coordinated, multi-wavelength campaign on M81*. Methods. The continuum emission from M81* was observed during three epochs with the IRAM Plateau de Bure Interferometer simultaneously at wavelengths of 3 and 1 mm. Results. We present the first flux measurements of M81* at wavelengths around 1 mm. We find that M81* is a continuously variable source with the higher variability observed at the shorter wavelength. Also, the variability at 3 and 1mm appears to be correlated. Like Sgr A*, M81* appears to display the strongest flux density and variability in the mm-to-submm regime. There remains still some ambiguity concerning the exact location of the turnover frequency from optically thick to optically thin emission. The observed variability time scales point to an upper size limit of the emitting region of the order 25 Schwarzschild radii. Conclusions. The data show that M81* is indeed a system with very similar physical properties to Sgr A* and an ideal bridge toward high luminosity AGN. The data obtained clearly demonstrate the usefulness and, above all, the necessity of simultaneous multi-wavelength observations of LLAGN.