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 −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%yr −1 and 0.24%yr −1 , corresponding to velocities at the tip of the X-ray jet of 7830-9200 km s −1 .

Evidence for past interaction with an asymmetric circumstellar shell in the young SNR Cassiopeia A

“…The discrepancies between models and observations might originate from the idealized and simplified description of the asymmetric shell adopted in our models. For instance, observations have shown some density enhancements in the north-east region (Weil et al 2020), where some evidence for deceleration of the forward shock was recently reported (Vink et al 2022). These findings suggest that the CSM structure may be more complex than modeled here.…”

“…Features similar to those observed by Sato et al (2018) might be obtained by rotating the asymmetric shell approximately 90 o clockwise about the y axis. In this case, however, we found that the models do not reproduce other asymmetries that characterize the reverse shock of Cas A, in particular the velocity profiles derived by Vink et al (2022) and the orientation of the offset between the geometric center of the reverse shock and that of forward shock (Gotthelf et al 2001). In Appendix A, we present an example of these models.…”

“…These velocities are somehow consistent with the remnant expanding through a spherically symmetric wind of the progenitor star and, in fact, are well reproduced by the models (e.g., Orlando et al 2016 and Paper I). The observations, however, suggest that the reverse shock in the southern and western quadrants of Cas A is stationary or is even moving inward in the observer frame toward the center of the explosion (e.g., Anderson & Rudnick 1995;Keohane et al 1996;DeLaney et al 2004;Helder & Vink 2008;Sato et al 2018) at odds with the model predictions (e.g., Vink 2020;Vink et al 2022; see also Paper I). In addition, observations of Cas A show an offset of ≈ 0.2 pc (at the distance of 3.4 kpc) between the geometric center of the reverse shock and that of forward shock (Gotthelf et al 2001) that cannot be reproduced by the models (see Paper I).…”

“…5, which shows the profiles of the forward and reverse shock velocities versus the position angle in the plane of the sky at the age of Cas A. For comparison, the figure also shows the profiles derived from a model not including the interaction of the remnant with the shell (upper left panel; model W15-2-cw-IIb-HD presented in Paper I) and the profiles derived from the analysis of Chandra observations of Cas A (black and magenta diamonds; Vink et al 2022). At the age of Cas A, the models describing the interaction with the shell show a forward shock that propagates with velocity between 5000 km s −1 and 6000 km s −1 at all position angles, thus producing results analogous to those derived from the model without the shell and in agreement with observations.…”

“…The above models, however, do not explain one of the most intriguing aspects of the Cas A structure, as evidenced by the analysis of the position and velocity of the forward and reverse shocks. Observations in different wavelength bands indicate a forward shock expanding with a velocity around ≈ 5500 km s −1 along the whole remnant outline (e.g., DeLaney & Rudnick 2003;Patnaude & Fesen 2009;Fesen et al 2019;Vink et al 2022) and a reverse shock moving outward with velocity ranging between 2000 km s −1 and 4000 km s −1 in the eastern and northern hemisphere of the remnant (e.g., Sato et al 2018;Fesen et al 2019;Vink et al 2022). These velocities are somehow consistent with the remnant expanding through a spherically symmetric wind of the progenitor star and, in fact, are well reproduced by the models (e.g., Orlando et al 2016 and Paper I).…”