Thermal emission from SNRs and plasma diagnostics

Abstract

We present here the observational results of Galactic SNRs focusing on their thin thermal emission. Their X-ray spectra give us a good diagnostics of the plasma. Generally, they cannot be fitted by using model spectra in collisional ionization equilibrium (CIE) condition. Some of them show gradients both in the temperature and in the abundance. The X-ray spectrum of young SNRs, like the Kepler's SNR, can be well fitted by the superposition of the fore shock component and the reverse shock component, both of which show gradients in various parameters: temperature, density, ionization parameter etc. Cassiopeia-A, one of the typical young SNR, shows not point symmetric but axial symmetric both in the intensity profile and in the line center energy profile. This suggests the Doppler motion of the plasma about several thousands km/sec. The Cygnus Loop, a middle-aged SNR, also showed gradients: the shell regions show low metal abundance and low temperature while the core region shows high metal abundance and high temperature. The plasma left in the core region must reflect the fossil of the supernova explosion. Based on the metal abundance, we conclude that the Cygnus Loop originated from the type II SN. The Rosat observation on the Vela SNR, another typical middle-aged SNR, revealed that there were several debris running over the shell which was formed by the strong shock propagated in the surrounding medium. ASCA confirmed the abundance anomaly suggesting that they were the fossil of the progenitor star. The plasma both in the young and in the middle-aged SNRs are in the mixing phase of the ejecta with the interstellar matter (ISM).