The X-ray structure and mass of the Cassiopeia A supernova remnant

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Abstract
The X-ray images of the Cassiopeia A supernova remnant from the Einstein Observatory have been processed by a maximum-entropy algorithm. The emission appears to originate in two concentric thin shells. We have used the image of highest resolution (about 4 arcsec) to derive temperature and density maps of the remnant assuming that the X-ray emission is thermal and from an optically thin hot gas. These maps are consistent with both pressure equilibrium and the overall X-ray spectrum. The mass of X-ray emitting gas is found to be at least 15 M⊙ which is considerably more than observed directly at other wavelengths. The X-ray structure and dynamics of Cas A are consistent with it being in a ‘free-expansion’ phase of evolution, with the bulk of the emission from a reverse shock in the ejecta. The progenitor star is likely to have been massive, as seems to be required by the element abundances of the optical knots.

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