The Evolution of Relativistic Electron Populations in Shell Supernova Remnants

By Martha Carol Anderson

Under the supervision of Dr. Lawrence Rudnick


Observational data regarding the acceleration of relativistic particles in shell-type supernova remnants is presented. As synchrotron spectral indices directly reflect the energy spectra of radiating particle populations, we have mapped the spatial distribution of spectral index in several shell SNRs. In particular, we address the question of whether bright, compact radio features in SNRs should be interpreted as sites of active particle acceleration, in analogy with studies of extragalactic radio jets (e.g. Meisenheimer et al. 1989).

We concentrate primarily on the young, well-studies SNR Cassiopeia A. As shock strength, hydrodynamic turbulence and acceleration efficiency are closely-linked subjects, we begin by constructing a dynamical picture describing the current evolutionary state of emission structures on both small and large spatial scales. The radio polarization properties of a remnant potentially hold important clues to its dynamical history, so we present a detailed study of polarimetric images of Cas A at lambda lambda(6) and 20 cm. To evaluate the dynamical status of various ejecta components in Cas A, the proper motions and brightness evolution of both the bulk radio ring and a sample of 304 compact radio features have been accurately determined from high-quality interferometric observations of the remnant at lambda lambda(6) and 20 cm, spanning a total time baseline of 12 years. We find that a single estimate of the ejecta expansion age is not appropriate, as the expansion derived from compact features varies azimuthally and radially in the remnant and differs from subsets of knots segregated by brightness.

These measurements suggest that, in Cas A, the deceleration of ejecta and radio emissivity are strongly couples. This is in agreement with numerical models of supersonic gaseous projectiles (Jones, Kang,Tregillis 1993) which show that the declaration of a clumpy ejectum is accompanied by the onset of dynamical instabilities which serve to amplify the local magnetic field, thereby enhancing synchrotron emissivity. An evolutionary sequence connecting optical-emitting knots to faint and then to bright radio knots is suggest by the data and models. Degree of deceleration appears to be the major factor discriminating between these three population of clumpy ejecta.

To facilitate a direct comparison between spectral and dynamical properties associated with small-scale radio features, we compute spectral indices for the same sample of knots used in the dynamical study and verify that significant spectral variations exist. We find that the spectral indices of compact radio features are uncorrelated with any dynamically important quantities (e.g., knot brightness, rate of brightness change, degree of deceleration). Spectral index shows a significant correlation only with the knots' (projected) radial positions within the remnant. This suggests that these radio-bright features are not themselves accelerating the relativistic electrons which illuminate them. It is more likely that these electrons have been entrained from the background into regions of high magnetic field associated with strongly decelerated clumps of ejecta. The spectral variations observed among compact features then must reflect modulations in the background particle energy spectra, perhaps instilled by temperature variations in the underlying thermal material. Comparisons with spectral variation found in Galactic remnants G39.2-0.3, G41.1-0.3, Kepler's SNR and with other previously published remnant studies are made.