The first measurement of Cassiopeia A's forward shock expansion rate

We have obtained a second-epoch observation of the Cassiopeia A supernova remnant (SNR) with the Chandra X-Ray Observatory to measure detailed X-ray proper motions for the first time. Both observations are 50 ks exposures of the ACIS-S3 chip, and they are separated by 2 years. Measurements of the thin X-ray continuum-dominated filaments located around the edge of the remnant (which are identified with the forward shock) show expansion rates from 0.02% to 0.33% yr^-1. Many of these filaments are therefore significantly decelerated. Their median value of 0.21% yr^-1 is equal to the median expansion of the bright ring (0.21% yr^-1) as measured with Einstein and ROSAT. This presents a conundrum if the motion of the bright ring is indicative of the reverse shock speed. We have also reevaluated the motion of the radio bright ring with emphasis on angle-averaged emissivity profiles. Our new measurement of the expansion of the angle-averaged radio bright ring is 0.07% ± 0.03% yr^-1, somewhat slower than the previous radio measurements of 0.11% yr^-1, which were sensitive to the motions of small-scale features. We propose that the expansion of the small-scale bright ring features in the optical, X-ray, and radio do not represent the expansion of the reverse shock, but rather represent a brightness-weighted average of ejecta passing through and being decelerated by the reverse shock. The motion of the reverse shock itself is then represented by the motion of the angle-averaged emissivity profile of the radio bright ring.