The forward and reverse shock dynamics of Cassiopeia A

We report on proper motion measurements of the forward- and reverse-shock regions of the supernova remnant Cassiopeia A (Cas A), including deceleration/acceleration measurements of the forward shock. The measurements combine 19 years of observations with the Chandra X-ray Observatory, using the 4.2-6 keV continuum band, preferentially targeting X-ray synchrotron radiation. The average expansion rate is $0.218 \pm 0.029$%yr$^{-1}$ for the forward shock, corresponding to a velocity of $\approx 5800$ km/s. The time derivative of the proper motions indicates deceleration in the east, and an acceleration up to $1.1\times 10^{-4}$ yr$^{-2}$ in the western part. The reverse shock moves outward in the East, but in the West it moves toward the center with an expansion rate of $-0.0225\pm 0.0007$ %yr$^{-1}$, corresponding to $-1884\pm 17$ km/s. In the West the reverse shock velocity in the ejecta frame is $\gtrsim 3000$ km/s, peaking at $\sim 8000$ km/s, explaining the presence of X-ray synchrotron emitting filaments there. The backward motion of the reverse shock can be explained by either a scenario in which the forward shock encountered a partial, dense, wind shell, or one in which the shock transgressed initially through a lopsided cavity, created during a brief Wolf-Rayet star phase. Both scenarios are consistent with the local acceleration of the forward shock. Finally we report on the proper motion of the northeastern jet, using both the X-ray continuum band, and the Si XIII K-line emission band. We find expansion rates of respectively 0.21%yr$^{-1}$ and 0.24%yr$^{-1}$, corresponding to velocities at the tip of the X-ray jet of 7830--9200 km/s.