On the X-ray spectrum of Kepler's supernova remnant

Abstract

We have devised a method to do nonequilibrium ionization calculations on the results of two-dimensional hydrodynamical simulations, based on the algorithm of Hughes & Helfand (1985). We have calculated the ionization structure and X-ray emission for a two-dimensional numerical hydrodynamical simulation for the remnant of Kepler's supernova (SN); the hydrodynamical model was presented in a previous paper. In this model, the progenitor of Kepler's SN is assumed to have been a massive runaway star ejected from the Galactic plane. In its red supergiant stage, its dense stellar wind was distorted and compressed into a bow shock by the ram pressure of the tenuous interstellar medium. The subsequent interaction of the supernova ejecta with this asymmetric circumstellar matter produced a strongly asymmetric supernova remnant (SNR). In this paper, we present calculated X-ray spectra for this hydrodynamical model. A comparison with observations implies a moderate overabundance of Fe in Kepler's SNR (only 50% larger than its cosmic value), in contrast to a large (6 to 15) Fe overabundance derived previously. However, we confirm earlier conclusions that Si and S abundances are 2 to 3 times solar. These modest enhancements of Si, S, and Fe may be attributed either to heavy-element enriched SN ejecta or to the initial chemical abundances of the SN progenitor, which originated in the metal-rich inner Galaxy. The comparison of our models with the observed spectra confirm theoretical predictions that moderate electron heating occurs at strong collisionless shock fronts, with the implied electron/mean temperature ratio of approximately 0.5.