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We have analyzed VLA polarization data on the supernova remnant Cas~A at multiple frequencies in the $\lambda\lambda$ 20cm and 6cm bands. This allows us to determine the rotation measures, depolarizations, and magnetic field orientations in the remnant.
We find that the depolarization is strongly anti-correlated with the X-ray emitting shell of thermal material. The relativistic plasma, especially the bright radio ring, is therefore found mostly within the thermal shell. At the same time, a number of blobs of relativistic plasma have ``broken out'' beyond this thermal shell, and show little depolarization. However, a wide range of observed rotation measures suggests that these break-out features are still accompanied by a substantial local thermal plasma.
Correcting for the rotation measures observed at high frequencies, we derive the direction of the remnant's magnetic field, and find it to be radial, to first order, as previously known. However, we find an additional, quasi-sinusoidal modulation of the field direction as a function of azimuthal angle, with an amplitude of approximately 30 degrees. This modulation can be simply modelled as due to the presence of the compressed local interstellar magnetic field within the relativistic plasma. This is not due to small scale clumps of interstellar plasma mixed with clumps of ejected plasma. Each relativistic electron actually finds itself in a local magnetic field that is the vector sum of the heavily amplified remnant field and the compressed ISM field. This result has important, but still not understood, consequences for the evolution of magnetic fields in Cas~A and other supernova remnants.
This work is supported in part by NSF grant AST 9100486 to the University of Minnesota.
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