The forward and reverse shock dynamics of Cassiopeia A
Abstract
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 0.029%yr-1 for the forward shock, corresponding to a velocity of ≈ 5800 km/s. 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. In the West the reverse shock velocity in the ejecta frame is 3000 km/s, peaking at 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.