Five years monitoring of extragalactic radio sources. III. Generalized shock models and the dependence of variability on frequency.
Abstract
Shocks propagating along the jets provide an unifying description for a wide variety of observed phenomena in active galactic nuclei. No complete and satisfactory physical models for the shock evolution can as yet be formulated. To provide a framework for comparison with observations, we present instead a generalized shock model, which incorporates the growth and the decay stages of the shock in a simple parametrized form. From our radio data we infer that the outbursts have similar spectra, and that their evolution also is so similar that a single evolutionary framework applies to them all. In this scenario the appearance of an outburst to an observer depends mainly on whether the shock reaches its maximal development at frequencies above the observing frequencies or below them. In the former case the character of the multifrequency flux variations is similar to that predicted by expanding source models, the outburst is unlikely to produce a detectable new cm- VLBI component, but may have an associated optical flare simultaneously with high radio frequency maxima. In the latter case the multifrequency lightcurves track each other with no time delays and similar amplitudes, the outburst is likely to be followed by an ejection of a new cm-VLBI component from the core, and is unlikely to have simultaneous optical events. In order to compare the model predictions with data, we calculate the maximum amplitude of variations at 37,22,14.5,8.0 and 4.8 GHz from our own and the University of Michigan monitoring data between 1980 and 1985 for 27 AGN. The dependence between {DELTA}S_max_ and ν is found to be in agreement with the shock scenario. We also compare some other model predictions with data.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- February 1992
- Bibcode:
- 1992A&A...254...71V