Theoretical X-Ray Spectra of Supernova Remnants in the Adiabatic Phase

Abstract Time development of thermal X-ray spectra of supernova remnants in the adiabatic phase of the blast-wave evolution is investigated, by assuming that the electron temperature is much lower than the ion temperature just behind the shock front. Attention is focused upon the effect of time-dependent ionization of the shock-heated interstellar gas. At early times in the adiabatic phase, the shocked gas cannot be ionized up to the equilibrium state. Consequently, line emission and two-photon emission are significantly enhanced from the equilibrium prediction in the energy range below 1 keV. On the other hand, bremsstrahlung and recombination radiation, which dominate the X-ray spectrum in the energy range above 2 keV, are less affected. Therefore, the time-dependent spectrum, when fitted with spectra of the isothermal plasmas in ionization equilibrium, is characterized by a two-component model. We suggest possibilities that the two-component appearance of the X-ray spectrum of Puppis A is due to the nonequilibrium effect and that the electron temperatures within Vela X and the Cygnus Loop are appreciably higher than have hitherto been estimated.