1994A&A...289...35V

Standard shocked jet models predict that the innermost radio core of an active galaxy consists of a relativistic jet and shocks in it. At high radio frequencies a simple two-component structure remains visible: the base of the jet and the most recent shock. These microarcsecond components are beyond the resolving power of present-day VLBI, but with well-sampled continuum monitoring their contributions to the total flux density can be separated. We have used Metsaehovi radio monitoring data at 22 and 37 GHz to derive distributions of jet flux density, shock flux density, variability timescales and linear sizes of shocks in a nearly complete 2 Jy sample of AGN. We find that the luminosity of the new shock component is usually comparable to that of the underlying jet. The derived intrinsic variability timescales indicate a surprisingly narrow range of characteristic linear sizes for shocks at the time of their 22/37 GHz luminosity maxima: 0.5-0.8 light-years, independent of redshift or source classification. Mapping the innermost core of a typical quasar at z≃0.5 requires a resolution of ≤50µas, a value within the reach of the RADIOASTRON and the VSOP space VLBI satellites.