Abstract In this paper, I discuss some historical data on the radio and X-ray variability of the active galaxy, Centaurus A (NGC 5128). The Cen A data reviewed herein were the first detection of concurrent radio and X-ray variability of an active galaxy. Such concurrent variability demands that the radio and X-ray light originate from the same region in the source, a result that allows us to further constrain the physical parameters in the emitting region. The radio and X-ray data from Cen A during this epoch bear a remarkable resemblence to both the radio data from 3C120 (and other AGN) and the radio data from galactic microquasars. The radio data for Cen A are not consistent with van der Laan expansion, a circumstance reminiscent of some of the time variability of the galactic microquasars. This suggests that concurrent, spatially resolved data from multifrequency campaigns will be critical to a refinement of source models for these objects, a result that motivates some comments on what we mean by concurrent, spatially resolved, multifrequency observations. Astrophysical jets are thus a remarkable laboratory: They provide a confirmation of special relativity in terms of relativistic Doppler boosting, superluminal motion, and time dilation effects. When coupled with their black hole neutron star origins, jets have implications for testing general relativity. As our understanding of the ubiquity of the jet phonomena has grown, we have been required to abandon the assumption of anisotropy in the emitting region in most but not all cases.

Keywords astrophysical jets active galactic nuclei quasars microquasars

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