[ITAL]Chandra[/ITAL] Detection of the Forward and Reverse Shocks in Cassiopeia A

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Abstract
We report the localization of the forward and reverse shock fronts in the young supernova remnant Cassiopeia A using X-ray data obtained with the Chandra X-Ray Observatory. High-resolution X-ray maps resolve a previously unseen X-ray feature encompassing the extremity of the remnant. This feature consists of thin, tangential wisps of emission bordering the outer edge of the thermal X-ray and radio remnant, forming a circular rim, ~27 in radius. Radio images show a sharp rise in brightness at this X-ray rim along with a large jump in the synchrotron polarization angle. These characteristics suggest that these wisps are the previously unresolved signature of the forward, or outer, shock. Similarly, we identify the sharp rise in emissivity of the bright shell for both the radio and X-ray line emission associated with the reverse shock. The derived ratio of the averaged forward and reverse shock radii of ~3 : 2 constrains the remnant to have swept up roughly the same amount of mass as was ejected; this suggests that Cas A is just entering the Sedov phase. Comparison of the X-ray spectra from the two shock regions shows that the equivalent widths of prominent emission lines are significantly lower exterior to the bright shell, as expected if they are respectively identified with the shocked circumstellar material and shocked ejecta. Furthermore, the spectrum of the outer rim itself is dominated by power-law emission, likely the counterpart of the nonthermal component previously seen at energies above ~10 keV.
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