The x-ray expansion of the supernova remnant Cassiopeia A

We present the first X-ray expansion measurements of the young, bright supe rnova 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 ta ken 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 fas t as the expansion of the bright radio ring and two-thirds as fast as the e nsemble of optical fast-moving knots. This argues that we are observing a f undamentally different hydrodynamical structure in each band, although all of these components are located on the same patchy ring. In addition, we de tect 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 mon otonically away from the center.