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

We have obtained a second epoch observation of the Cassiopeia A supernova remnant with the Chandra X-ray Observatory to measure detailed X-ray proper motions for the first time. Measurements of the X-ray continuum dominated filaments located around the edge of the remnant show expansion rates from 0.02% to 0.33% yr ^-1 . Their median value of 0.21% yr ^-1 is equal to the median expansion of the bright ring as measured in X-rays. We have also re-evaluated 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 , 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 is represented by the motion of the angle-averaged emissivity profile of the radio bright ring.