A comparison of X-ray and radio emission from the supernova remnant Cassiopeia A

We compare the radio and soft X-ray brightnesses as a function of position within the young supernova remnant Cassiopeia A. A moderately strong correlation (r = 0.7) was found between the X-ray emission (corrected for interstellar absorption) and radio emission, showing that the thermal and relativistic plasmas occupy the same volumes and are regulated by common underlying parameters. The logarithmic slope of the relationship, In S_x = 1.21nS_radio + In k, implies that the variations in brightness are primarily due to path length differences. The X-ray and radio emissivities are both high in the same general locations, but their more detailed relationship is poorly constrained and probably shows significant scatter. The strongest radio and X-ray absorption is found at the western boundary of Cas A. Based on the properties of Cas A and the absorbing molecular cloud, we argue that they are physically interacting. We also compare column densities derived from the Advanced Satellite for Cosmology and Astrophysics with 21 cm H I and 18 cm OH optical depths in the direction of Cas A, in order to provide an independent estimate of interstellar medium properties. We derive an average value for the H I spin temperature of ≈40 K and measure the ratio OH/H₂, which is nominally larger than previous estimates.