Ultracompact jets in active galactic nuclei

We study the properties of ultracompact jets in several prominent radio sources (Cygnus A, 3C 309.1, 3C 345, 3C 395, 4C 39.25, and 1038+528A), using the frequency dependence of observed position of the optically thick jet core. Frequency dependent offsets of the core positions are used for calculating the luminosities, magnetic fields, and geometrical properties of the jets. Opacity effects are studied, for both synchrotron self–absorption and external free–free absorption. Pressure and density gradients in the jets and in surrounding ambient medium are shown to be a primary factor determining the observed properties of ultracompact jets. We discuss possible applications of opacity effects to studying the conditions existing in central regions of active galactic nuclei.