High-resolution, multi-wavelength, observations are important because the radio spectrum constrains the energy spectrum and spatial distribution of the relativistic particles. The key is to relate measured spectral variations to the dynamical structure of the remnant, since models of particle acceleration in SNRs, either by shocks or by second-order Fermi (stochastic) acceleration in interior turbulence, predict structure in the particle distributions produced. Measured variations must be related to acceleration processes or the energy spectrum of the seed particles. Variations in older SNRs can also be related to compression of CR gas and IS magnetic field.
Only a handful of SNRs can usefully be studied in this way with the current VLA-spatially-resolved studies of the spectral index between 1.4 and and 4.9 GHz have been possible only on a few remnants. Dispersions in spectral index of up to 0.3 have been reported, though other reported variations are less certain. The key limitations in extending the accuracy of these studies to other SNRs have been insufficient surface brightness sensitivity at higher frequencies and insufficient angular resolution at lower frequencies.
Of Galactic SNRs which can be seen with the VLA, the complete structure of only -10 can be reliably imaged with the VLA at 6cm. The E configuration plus total power measurements will increase by more than a factor of 2 the number of SNRs that can be studied at 6cm. Furthermore some studies could be extended to 3.6cm for at least several bright shell-type SNRs in addition to Cas A.
Observations at 333 MHz with sufficient sensitivity to trace the variations have only been available recently with the advent of wide field imaging. The range of a factor of 12 from 333 MHz to 4.9 GHz should produce much more robust results, as well as allowing three frequency spectral-index maps to be constructed and cross-checked against one another. However while the present angular resolution at 333 MHz ( in the A configuration) will be useful for following larger scale variations, observations at resolution with the A+ configuration are required to properly extend scaled array observations from 4.9 and 1.4 GHz to lower frequencies. Completion of the 74 MHz system and its extension to the A+ configuration and inner VLBA antennas would provide yet a further extension of spectral index studies in frequency space. Because cm-wave observations are not sensitive to electron energies associated with particle injection, important aspects of the acceleration mechanism cannot be directly addressed at those wavelengths. Below 100 MHz, the ability to measure the mid-energy range of synchrotron emitting electrons, where injection may be taking place, is greatly improved. There is therefore a real possibility of probing the acceleration process with 74 MHz VLA observations.
Reverse-shock related emission plays an important rôle in young SNRs such as Cas A. X-rays are generated when freely expanding ejecta is heated by the reverse shock. This mechanism has been used to explain the inner components of the concentric double-shell X-ray morphology seen in Tycho and Cas A. However, information on the physical conditions of the unshocked ejecta interior to SNR reverse shocks has been scant, as that cooler material has been largely unobservable. Recent 74 MHz VLA data indicate free-free absorption of low frequency radio emission by ionized gas interior to the reverse shock in Cas A. This provides the first evidence for the inner component of cool, ionized ejecta in Cas A expected by theory. The absorption is manifest as a large, arc-minute scale spectral index flattening. Emission from the back face of the synchrotron emitting radio shell provides the background against which the free-free absorption at 74 MHz is revealed. The present 8 antenna 74 MHz prototype has insufficient sensitivity to extend such studies to other SNRs other than, perhaps, the Crab Nebula. And even here the u-v coverage is so poor that observations over many years must be obtained to achieve adequate u-v coverage, i.e., the system is prohibitively slow. A full 27 antenna system would extend meaningful studies to other SNRs.