Search

X‐ray observations of the Cassiopeia A supernova remnant reveal explosive nucleosynthesis products such as Si and Fe, and thus provide a unique window into the core‐collapse explosion that formed the remnant 330 years ago. We review current progress using X‐ray spectra extracted on arcsecond angular scales from a 106 s Chandra observation of Cas A, in conjunction with models that follow the remnant’s hydrodynamical evolution and treat the relevant plasma microphysics. We address questions related to the explosion such as the degree of explosion asymmetry, the nature of the jets, the nature of the circumstellar environment, and extent of radial mixing of the Fe ejecta.

We discuss some or our favorite open issues in ionization balance. In particular, we are concerned with steady‐state ionization balance in the presence of thermal instability, and in time dependent problems. In both cases, having accurate rate coefficients for the ionization and recombination processes is crucial to modeling the plasma state correctly, since when instabilities are involved, small errors in input physics can translate to large errors in results.