A&A 463, 551-557 (2007)
DOI: 10.1051/0004-6361:20066383

The millimetre variability of M 81*

Multi-epoch dual frequency mm-observations of the nucleus of M 81

R. Schödel¹, M. Krips², S. Markoff³, R. Neri⁴, and A. Eckart<sup>1

1</sup>  I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
    e-mail: [rainer;eckart]@ph1.uni-koeln.de
²  Harvard-Smithsonian Center for Astrophysics, SMA project, 645 North A'ohoku Place, Hilo, HI 96720, USA
    e-mail: mkrips@cfa.harvard.edu
³  Sterrenkundig Instituut "Anton Pannekoek", Universiteit van Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
    e-mail: sera@science.uva.nl
⁴  Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d'Hères, France
    e-mail: neri@iram.fr

(Received 12 September 2006 / Accepted 21 November 2006)

Abstract
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. M 81*, the nucleus of the nearby spiral galaxy M 81, is an ideal source to extend the insights obtained on Sgr A* toward higher luminosity AGN. Here we present observations at 3 and 1 mm that were obtained within the framework of a coordinated, multi-wavelength campaign on M 81*.
Methods.The continuum emission from M 81* 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 M 81* at wavelengths around 1 mm. We find that M 81* is a continuously variable source with the higher variability observed at the shorter wavelength. Also, the variability at 3 and 1 mm appears to be correlated. Like Sgr A*, M 81* 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 M 81* 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.

Key words: accretion, accretion disks -- galaxies: active -- galaxies: jets -- radio continuum: galaxies

© ESO 2007

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.