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The observation of superluminal motion in extragalactic radio jets has spurred the notion that these jets are moving relativistically. Doppler boosting due to relativistic motion has also been used to explain the observed one-sidedness of these jets, and the high surface brightnesses encountered (Kellermann and Pauliny-Toth 1981, Zensus and Pearson 1990, Zensus and Kellermann 1994, Cawthorne 1991). The symmetric twin-relativistic jet model (STRJM) is an integral part of theories concerning the unification of powerful radio galaxies and quasars (e.g., Barthel et al. 1989, Antonucci 1993). Cygnus A provides an important test, since the jets in Cygnus A are thought to be close to the sky plane, as deduced from the large-scale morphology of the source (Hargrave and Ryle 1974, Scheuer 1983). The predictions are that for such a source the jet motion will be trans-luminal and the ratio of flux density in the jet to that in the counterjet will not be extreme.

The first VLBI images of Cygnus A revealed a pc-scale core source with a slight elongation along the axis of the kpc-scale source (Kellermann et al. 1981, 1975). This elongation was resolved into a core-jet by Linfield (1981) of approx 4 mas length. Further VLBI imaging revealed a `knotty' jet extending 20 mas from the core source at the same position angle as the kpc-scale jet, and proper motion for one of the jet components was measured (Carilli et al. 1989b, 1994a).

The most recent VLBI studies of Cygnus A are by Sorathia et al. (1996) and Krichbaum et al. (1996). Both these studies detect a counterjet in Cygnus A. Sorathia et al. use three epochs of observations to determine the apparent velocity, betaA (in units of c), of the outermost jet component (J4) to be: betaA = 0.55 ± 15 h-1, and they find a value for the ratio of the integrated flux density in the jet to that in the counterjet, R, of: R = 5 ± 2 at 5 GHz. Using these two parameters (betaA and R), they apply the equations for the STRJM (Scheuer and Readhead 1979) to constrain the jet angle relative to our line of sight, theta, and to constrain the true jet velocity, beta:

betaA = beta sintheta / (1 - beta costheta)


R = ((1 + beta costheta) / (1 - beta costheta))2-alpha

For h = 1/2 they find 1.0 geq beta geq 0.6 and 85° geq theta geq 50°. Hence, the jet in Cygnus A must be at an intermediate angle relative to our line of sight, and moving at a mildly relativistic velocity.

The higher resolution observations of Krichbaum et al. (1996) present a somewhat more complicated picture. The complexity and temporal variability of the sub-pc-scale structure makes core idenfication difficult. Making a nominal core identification with the most inverted spectrum component, Krichbaum et al. find evidence that the inner jet components may have a range in apparent velocities (betaA between 0.2 and 0.8 h-1). This would be similar to the well studied case of the jet in M 87 for which knot velocities ranging from sub- to superluminal have been observed (Biretta et al. 1995). Such an observation suggests that the apparent velocities represent a pattern speed, eg. of surface waves on the jet, and not the true jet fluid velocity, thereby invalidating the standard STRJM analysis.

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