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 epoch observations are 50 ks exposures of the ACIS-S3 chip and they are separated by 2 years. Measurements of the thin Xray continuum dominated filaments located around the edge of the remnant (that are identified with the forward shock) show expansion rates from 0.02% yr to 0.33% yr. Many of these filaments are therefore significantly decelerated. Their median value of 0.21% yr is equal to the median expansion of the bright ring (0.21% yr) 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 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, somewhat slower than the previous radio measurements of 0.11% yr 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. Subject headings: ISM: supernova remnants — ISM: individual (Cassiopeia A)