Second epoch VLBI observations of the nuclear jet in Cygnus A: Subluminal jet proper motion measured

Abstract

We present an image of the core and nuclear jet in Cygnus A at 5 GHz with a resolution of ~1 mas, obtained from "global array" MKIIIA VLBI observations. Comparison of this image (epoch 1991.44) with a previous image made using similar techniques (epoch 1986.89) reveals outward motion of the brighter features in the jet between epochs. The allowed range in velocity for the brightest feature in the jet is: 0.19 c h^-1^ to 0.48 c h^-1^. The peak surface brightness and total flux density of the core and jet have varied < 10% between epochs, although individual knots may have varied by as much as 40%. The pc-scale jet is aligned with the kpc-scale jet to within 20. The core is self-absorbed, with a steeply rising spectrum of index >=0.9 between 5 and 43 GHz. The counterjet in Cygnus A remains elusive. From a combined epoch image we determine a nominal "best" range for the jet to counterjet flux density ratio of: 8 <= R <= 21, with a firm lower limit being: R >= 5, as dictated by extensive analysis of "simulated" data sets. In the context of the twin- symmetric relativistic jet model for powerful radio galaxies and quasars, the conclusions are that the Cygnus A jet must be at an intermediate angle relative to our line of sight (between 11^deg^ and 60^deg^), and must be moving at an intermediate velocity (between 0.37 and 0.74 times the speed of light).