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 dynamic range (232 ks) image reveals low surface brightness X-ray structure that appears qualitatively similar to corresponding radio features. We consider the correlation between the X-ray and radio morphologies and its physical implications. After correcting for the inhomogeneous absorption across the remnant, we performed a point-by-point (4'' resolution) surface brightness comparison between the X-ray and radio images. We find a strong (r=0.75) log-log correlation, implying an overall relationship of log(ΣX-ray) ∝ (2.21 ± 0.05) × log(Σradio). This is consistent with proportionate partition (and possibly equipartition) between the local magnetic field and the hot gas—implying that Cas A's plasma is fully turbulent and continuously amplifying the magnetic field.