On energy-dependent propagation effects and acceleration sites of relativistic electrons in Cassiopeia A

We consider the effect of energy dependent propagation of relativistic electrons in a spatially inhomogeneous medium in order to interpret the broad-band nonthermal radiation of the young shell-type supernova remnant (SNR) Cassiopeia A. A two-zone model is proposed that distinguishes between compact, bright steep-spectrum radio knots and the bright fragmented radio ring on the one hand, and the remainder of the shell - the diffuse `plateau' - on the other hand. In the framework of this model it is possible to explain the basic features of the spectral and temporal evolution of the synchrotron radiation of Cas A if one assumes that these compact structures correspond to sites of efficient electron acceleration producing hard spectra of accelerated particles with power-law indices beta_acc ~ 2.2. The resulting energy distribution of radio electrons in these compact structures becomes significantly steeper than the electron production spectrum on timescales of the energy dependent escape of these electrons into the surrounding diffuse plateau region. We argue that the steepness, rather than the hardness, of the radio spectra of compact bright structures in clumpy sources can in general be considered as a typical signature of sites where strong electron acceleration has built up high gradients in the spatial distribution of radio electrons. Subsequent diffusive escape then modifies their energy distribution, leading to potentially observable spatial variations of spectral indices within the radio source. Qualitative and quantitative interpretations of a number of observational data of Cas A are given. Predictions following from the model are discussed.