Supernova remnants in the Sedov expansion phase: Thermal X-ray emission

Improved calculations of X-ray spectra for supernova remnants (SNRs) in the Sedov-Taylor phase are reported, which for the first time include reliable atomic data for Fe L-shell lines. This new set of Sedov models also allows for a partial collisionless heating of electrons at the blast wave and for energy transfer from ions to electrons through Coulomb collisions. X-ray e mission calculations are based on the updated Hamilton-Sarazin spectral mod el. The calculated X-ray spectra are successfully interpreted in terms of t hree distribution functions : the electron temperature and ionization times cale distributions, and the ionization timescale-averaged electron temperat ure distribution. The comparison of Sedov models with a frequently used sin gle nonequilibrium ionization (NEI) timescale model reveals that this simpl e model is generally not an appropriate approximation to X-ray spectra of S NRs. We find instead that plane-parallel shocks provide a useful approximat ion to X-ray spectra of SNRs, particularly for young SNRs. Sedov X-ray mode ls described here, together with simpler plane shock and single-ionization timescale models, have been implemented as standard models in the widely us ed XSPEC v11 spectral software package.