THE POLARIZATION AND DEPOLARIZATION OF RADIO-EMISSION FROM SUPERNOVA REMNANT CASSIOPEIA-A

We present a study of polarimetric images of the supernova remnant Cassiopeia A at observing wavelengths of lambda = 6 and 20 cm. We find that the bright ring is strongly depolarized at lambda = 20 cm and conclude that this is due to a mixing of the relativistic plasma responsible for the synchrotron emission and the thermal soft X-ray-emitting material behind the reverse shock. On large scales, the magnetic field is radial in Cas A, as is common in young remnants. However, the radial field region extends well beyond the bright ring, putting constraints on the field-generating mechanism. Two interesting types of small-scale features are also found and probably reflect the same underlying phenomena; the first are features at lambda = 20 cm that appear to have penetrated beyond the Faraday depolarizing shell, and the second are regions of small-scale variations in the magnetic field direction at lambda = 6 cm. Both of these are likely to be identified with moderately dense clumps of stellar ejecta, leading to the bow shocks discussed by Braun, Gull, & Perley (1987) and the evolving compact features discussed by Anderson & Rudnick (1995a).