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Emission from reverse shocks plays an important role in the radiation from young supernova remnants such as Cas-A. X-ray emission is generated when freely-expanding ejecta is heated by the reverse shock, and 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 regions interior to the reverse shocks has been scant, as that cooler material has been considered largely unobservable. A notable acception are the Fe II absorption features seen in the UV spectrum towards the center of SN 1006, thought to be related to the photoionization of unshocked ejecta by the X-ray emission from the reverse shock.
In this paper we present 74 and 330 MHz VLA data which indicate the presence of free-free absorption of low frequency radio emission interior to the reverse shock in Cas-A. This provides the first evidence for the inner component of cool, ionized gas in Cas-A which would be expected based on our current understanding of reverse shocks in young SNRs. The absorption is manifest as a large, arc-minute scale spectral index flattening and is therefore unlikely to be related to the smaller, arc-second scale radio spectral index variations reported at higher frequencies. The latter, while poorly understood, is believed related to the details of the shock acceleration processes which give rise to the bright, synchrotron emitting radio shell located well beyond the reverse shock, coincident with Cas-A's outer X-ray shell. Emission from the back face of this radio shell provides the background against which the free-free absorption is revealed.
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