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 yr 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 ± 0.029% yr^-1 for the forward shock, corresponding to a velocity of ≍5800 km s^-1. The time derivative of the proper motions indicates deceleration in the east, and an acceleration up to 1.1 × 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 ± 0.0007 % yr^-1, corresponding to -1884 ± 17 km s^-1. In the west, the reverse shock velocity in the ejecta frame is ≳3000 km s^-1, peaking at ~8000 km s^-1, 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% and 0.24% yr-1, corresponding to velocities at the tip of the X-ray jet of 7830-9200 km s^-1.