Evidence for thermal absorption inside Cassiopeia A

Abstract

Subarcminute resolution images of Cas A at 74 and 333 MHz are presented. Comparison with each other and with higher frequency observations indicates that the spectral index of its nonthermal emission flattens below 333 MHz and that the effect is confined toward the central 1' of the source and is maximum at its center. The source of the flattening is unknown, although we show that the data are consistent with absorption by ionized gas inside the radio shell. Such a component may be related to unshocked ejecta still freely expanding within the boundaries of the reverse shock as delineated by X-ray observations.