Chandra detection of the forward and reverse shocks in Cassiopeia A

We report the localization of the forward and reverse shock fronts in the y oung supernova remnant Cassiopeia A using X-ray data obtained with the Chan dra X-Ray Observatory. High-resolution X-ray maps resolve a previously unse en X-ray feature encompassing the extremity of the remnant. This feature co nsists of thin, tangential wisps of emission bordering the outer edge of th e thermal X-ray and radio remnant, forming a circular rim, similar to2.'7 i n radius. Radio images show a sharp rise in brightness at this X-ray rim al ong with a large jump in the synchrotron polarization angle. These characte ristics suggest that these wisps are the previously unresolved signature of the forward, or outer, shock. Similarly, we identify the sharp rise in emi ssivity of the bright shell for both the radio and X-ray line emission asso ciated with the reverse shock. The derived ratio of the averaged forward an d reverse shock radii of similar to3 : 2 constrains the remnant to have swe pt up roughly the same amount of mass as was ejected; this suggests that Ca s 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 emissio n lines are significantly lower exterior to the bright shell, as expected i f they are respectively identified with the shocked circumstellar material and shocked ejecta. Furthermore, the spectrum of the outer rim itself is do minated by power-law emission, likely the counterpart of the nonthermal com ponent previously seen at energies above similar to 10 keV.