SITES OF RELATIVISTIC PARTICLE-ACCELERATION IN SUPERNOVA REMNANT CASSIOPEIA-A

We have determined the synchrotron spectral indices of 304 compact rad io knots in the supernova remnant Cassiopeia A. A comparison of these data with the dynamical and brightness properties of these knots tabul ated by Anderson & Rudnick indicates that spectral index shows a signi ficant correlation with projected radius from the center of the remnan t. Spectrally flat knots reside in a shell coincident with the bright radio ring, while steeper knots occupy a shell coincident with the dif fuse radio plateau surrounding the ring. To a lesser extent, we find s pectral index to be correlated also with the radio brightness of the k not, in the sense that brighter knots tend to have steeper spectra. No significant correlation is found between spectral index and knot dece leration or rate of brightness change. As the synchrotron spectral ind ex traces the distribution of energy among relativistic particle popul ations, we use these results to study the nature of particle accelerat ion mechanisms active in Cas A. Given the dual-shell nature of the spe ctral index distribution in Cas A and the lack of strong correlation b etween spectral index and dynamical properties of the knots, we conclu de that radio-bright compact features are not sites of currently activ e particle acceleration in Cas A. This conclusion is in agreement with models of supersonic gaseous bullets constructed by Jones, Kang, & Tr egillis. In these models, the marked synchrotron brightening which acc ompanies bullet deceleration is due primarily to preexisting relativis tic particles radiating in rapidly amplifying shear-layer magnetic fie lds, rather than a large infusion of new relativistic particles accele rated in situ. Spectral variations between compact features in Cas A a re more likely to reflect modulations in the background particle energ y spectra within the remnant, perhaps instilled by temperature variati ons in the underlying thermal material. This interpretation requires t hat the diffuse synchrotron emission show the same spectral variations as seen in compact features; preliminary indications suggest that thi s is the case.