A Stationary Core with a One‐sided Jet in the Center of M81

The nucleus of M81 was observed at 8.3 GHz with VLBI at 20 epochs over4.5yrs, with a linear resolution at the source of about 2000 AU or 0.01 pc.Phase-referenced mapping with respect to the geometric center of supernova1993J enabled us to find, with a standard error of about 600 AU, a stationarypoint in the source. We identify this point as the location of the core and theputative black hole at the gravitational center of the galaxy. The upper boundon the core's average velocity on the sky is <730 km/s. A short, one-sided jetextends from the core. The orientation of the jet varies smoothly, withtimescales of about 1yr and an rms of 6degrees about the mean of 50degrees.Occasionally the jet appears to bend to the east. The length of the jet is onlyabout 1 mas (3,600 AU), and varies with an rms of about 20%. The inferredspeeds are below 0.08c. The total flux density of the core-jet varieserratically, changing on occasion by a factor of two over a few weeks, withoutany significant changes in the source size and orientation. The inferredvelocity of the plasma flow is >0.25c. The results are consistent with a modelin which plasma condensations with short lifetimes are ejected relativisticallyfrom the core on a timescale of less than a few weeks, and then travel along atube whose pattern and geometry are also variable but only on a timescale ofabout one year. The central engine of M81 has qualitative similarities to thoseof powerful the AGN of radio galaxies and quasars, and may also represent inpower and size a scaled-up version of the largely hidden nucleus in our ownGalaxy.Comment: 31 pages (including Figures); Accepted for publication in the Astrophysical Journa