The X-Ray Expansion of the Supernova Remnant Cassiopeia A

We present the first X-ray expansion measurements of the young, bright supernova remnant Cas A. We have acquired a deep (232 ks) ROSAT high-resolution image of Cas A that is compared with archival images of similar quality taken 17 yr earlier with the Einstein Observatory . The average expansion rate between the two epochs is found to be 0.20%±0.01% yr -1 , twice as fast as the expansion of the bright radio ring and two-thirds as fast as the ensemble of optical fast-moving knots. This argues that we are observing a fundamentally different hydrodynamical structure in each band, although all of these components are located on the same patchy ring. In addition, we detect significant variations in the expansion rate as a function of azimuth around the bright X-ray ring. These results are discussed in the context of two classes of models, one involving a preexisting circumstellar shell and the other having a uniform or gradiated density profile that decreases monotonically away from the center.