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.
Bibliographical noteFunding Information:
3The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
SNR research at the University of Minnesota is supported by the NASA Graduate Research Program and the National Science Foundation under grant AST 96-19438. We would like to thank T. W. Jones and S. P. Reynolds for discussions about SNR dynamics and comments on the Letter and R. Petre for assistance with the ROSAT observations.
- ISM: Individual (Cassiopeia A)
- ISM: Kinematics and dynamics
- Supernova remnants