A RELATION BETWEEN THE APPARENT VELOCITY AND THE SPECTRAL POWER OF SUPERLUMINAL RADIO-SOURCES

It is proved that the apparent transverse velocities beta(app) and the spectral powers at 10 GHz P10 of the cores of known superluminal sources are correlated. An interpretation of this result within the framework of the relativistic jet model shows that the jet Lorentz factor gamma depends on the intrinsic luminosity of the sources. The probable existence of two subpopulations of the superluminal sources is shown by a linear regression analysis. "Small" sources, i.e. the sources without extended emission or with a distance from the core to the outer edge of the lobe <20 kpc, show lower beta(app) than the "large" objects; this can be interpreted as an orientation effect. Since all BL Lac objects belong to the "small" class, this result sheds new light on such a question as why these sources have lower apparent velocities than the "classical" superluminal quasars.