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 approximate to 5800 km s(-1). The time derivative of the proper motions indicates deceleration in the east, and an acceleration up to 1.1 x 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 greater than or similar to 3000 km s(-1), peaking at similar to 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).

Automated summary

We report proper motion measurements of the forward- and reverse shock regions of supernova remnant Cassiopeia A (Cas A), revealing the deceleration/acceleration of the forward shock and the outward/inward motion of the reverse shock. The findings support the presence of X-ray synchrotron emitting filaments and propose two scenarios explaining the backward motion of the reverse shock.