Evidence for proportionate partition between the magnetic field and hot gas in turbulent Cassiopeia A

We present a deep X-ray observation of the young Galactic supernova remnant Cassiopeia A, acquired with the ROSAT High Resolution Imager. This high dy namic range (232 ks) image reveals low surface brightness Xray structure th at appears qualitatively similar to corresponding radio features. We consid er the correlation between the X-ray and radio morphologies and its physica l implications. After correcting for the inhomogeneous absorption across th e remnant, we performed a point-by-point (4 " resolution) surface brightnes s comparison between the Xray and radio images. We find a strong (r = 0.75) log-log correlation, implying an overall relationship of log (Sigma(X-ray) ) proportional to (2.21 +/- 0.05) x log(Sigma(radio)). This is consistent w ith proportionate partition land possibly equipartition) between the local magnetic field and the hot gas-implying that Cas A's plasma is fully turbul ent and continuously amplifying the magnetic field.