The nuclear jet in M81

In this Letter we apply the jet-disk symbiosis model developed for Sagittarius A* to M81*-the nucleus of the nearby galaxy M81. The model accurately predicts the radio flux and size of M81* for the observed bolometric luminosity of the nuclear source, with no major free parameter except for the inclination angle. We point out that the usually applied, free conical jet emission model implies a longitudinal pressure gradient that must lead to a moderate acceleration of the jet along its flow direction. This usually neglected, gradual acceleration naturally accounts for the inverted spectrum and the size-frequency relation of M81*, and may be a general feature of radio cores. So far, M81* is the best case for a radio-loud jet nature of the compact radio core in the nucleus of a spiral galaxy. The fact that one can account for Sgr A* and M81* with the same model by simply changing the accretion rate strongly supports the jet-disk symbiosis model as an explanation for the compact radio cores of galaxies in general.