Broadband optical and narrowband Si xiii X-ray images of the young Galactic supernova remnant Cassiopeia A ( Cas A ) obtained over several decades are used to investigate spatial and temporal emission correlations on both large and small angular scales .
The data examined consist of optical and near infrared ground-based and Hubble Space Telescope images taken between 1951 and 2011 , and X-ray images from Einstein , ROSAT , and Chandra taken between 1979 and 2013 .
We find weak spatial correlations between the remnant ’ s X-ray and optical emission features on large scales , but several cases of good optical/X-ray correlations on small scales for features which have brightened due to recent interaction with the reverse shock .
We also find instances where : ( i ) a time delay is observed between the appearance of a feature ’ s optical and X-ray emissions , ( ii ) displacements of several arcseconds between a feature ’ s X-ray and optical emission peaks and , ( iii ) regions showing no corresponding X-ray or optical emissions .
To explain this behavior , we propose a highly inhomogeneous density model for Cas A ’ s ejecta consisting of small , dense optically emitting knots ( n \sim 10 ^ { 2 - 3 } cm ^ { -3 } ) and a much lower density ( n \sim 0.1 - 1 cm ^ { -3 } ) diffuse X-ray emitting component often spatially associated with optical emission knots .
The X-ray emitting component is sometimes linked to optical clumps through shock induced mass ablation generating trailing material leading to spatially offset X-ray/optical emissions .
A range of ejecta densities can also explain the observed X-ray/optical time delays since the remnant ’ s \approx 5000 km s ^ { -1 } reverse shock heats dense ejecta clumps to temperatures around 3 \times 10 ^ { 4 } K relatively quickly which then become optically bright while more diffuse ejecta become X-ray bright on longer timescales .
Highly inhomogeneous ejecta as proposed here for Cas A may help explain some of the X-ray/optical emission features seen in other young core collapse SN remnants .