The evolution of relativistic electron populations in shell supernova remnants

Observational data regarding the acceleration of relativistic particles in shell-type supernova remnants (SNRs) 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 should be interpreted as sites of active particle acceleration, in analogy with studies of extragalactic radio jets.

We concentrate primarily on the SNR Cassiopeia A and begin by constructing a dynamical picture describing the current evolutionary state of emission structures on both small and large spatial scales. To this end, 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 the expansion timescales derived for sets of compact features varies azimuthally and radially within the remnant and differs for subsets of knots segregated by brightness.

These measurements suggest that, in Cas A, the deceleration of ejecta and radio emissivity are strongly coupled. This is in agreement with numerical models of supersonic gaseous projectiles which show that the deceleration of a clumpy ejectum is accompanied by the onset of dynamical instabilities which serve to amplify the local magnetic field, thereby enhancing synchrotron emissivity.

We find that the spectral indices of compact radio features in Cas A 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. Comparisons with spectral variations found in Galactic remnants G39.2-0.3, G41.1-0.3, Kepler's SNR and with other previously published remnant studies are made.