The low frequency structure of Cassiopeia

The general structure of the supernova remnant Cassiopeia A is described, with particular reference to the optical jet of fast moving knots, the low frequency anomaly and previous VLBI results. The implementation of a three station interferometer system, measuring the visibility function of the nebula at 38MHz along a mean position angle of 83°, is described; both the closure results and the reconstructed visibility function are presented, from which a significant asymmetry in the source structure is evident. The results are interpreted by Fourier transforming the weighted visibility data, regu­larly gridded by interpolation of the reconstructed visibility function. The resultant, convolved brightness distribution of the nebula is compared with distributions synthesized to the same resolution, from the results of Jennison and Latham, and Matheson.

A model fitting procedure is described, based on a two component, four parameter model, from which it is concluded that the results are consistent with a symmetri­cal nebula of radius 135±3 arcsec. with a jet, of indeter­minate width, containing between 4% to 6% of the total flux at a distance of 200±20 arcsec. east of the source phase centre.

The remarkable coincidence of the point source solutions of the model fitting procedure with some of Hutton et. al.'s results at slightly higher frequencies is pointed out, and the likelihood of a relationship between the new asymmetrical component and the optical and X-ray jet is commented on. A brief discussion of the astrophysical consequences of the asymmetrical component follows, with reference to a possible association between the jet and the low frequency anomaly.