The X-ray line emission from the supernova remnant W49B

The Galactic supernova remnant W49B has one of the most impressive X-ray em ission-line spectra obtained with ASCA. We use both plasma line diagnostics and broadband model fits to show that the Si and S emission lines require multiple spectral components. The spectral data do not necessarily require individual elements to be spatially stratified, as suggested by earlier wor k, although when ASCA line images are considered, it is possible that Fe is stratified with respect to Si and S. Most of the X-ray emitting gas is fro m ejecta, based on the element abundances required, but is surprisingly clo se to being in collisional ionization equilibrium. A high ionization age im plies a high internal density in a young remnant. The fitted emission measu re for W49B indicates a minimum density of 2 cm(-3), with the true density likely to be significantly higher. W49B probably had a Type Ia progenitor, based on the relative element abundances, although a low-mass Type II proge nitor is still possible. We find persuasive evidence for Cr and possibly Mn emission in the ASCA spectrum-the first detection of these elements in X-r ays from a cosmic source.