X-ray Study of Ionization States in Supernova Remnants,

Masahiro Kawasaki

In this thesis, we present the results of systematic study on ionization states of the plasma in supernova remnants (SNRs) using the data obtained by the ASCA and XMM-Newton satellites.

The plasma in an SNR is so thin (~1cm-3) that it takes 104-5yr to reach collisional ionization equilibrium with electrons heated to ~107K by the shock wave. Therefore, X-ray investigation of ionization state of the plasma in an SNR provides us a strong suggestion to the evolutionary stage of the remnant. We analyzed ionization states of six "mixed-morphology" SNRs (MM SNRs), IC 443, W49B, W28, W44, 3C391, and Kes 27, which show center-filled thermal X-ray emission unlike the shell-like radio morphology. It is hard to explain their X-ray features by the standard model of SNR evolution (e.g., the Sedov solution). Thus, our study is valid for MM SNRs since the ionization state can be derived independently of the X-ray morphology. We estimated the ionization degree of the SNR directly from the X-ray spectrum by comparing the continuum shape and intensity ratios of the H-like Kα line to the He-like Kα line of heavy elements, which is possible with the ASCA SIS detector.

We found highly ionized plasmas to reach ionization equilibrium in the all MM SNRs, which is systematically more ionized than the underionized plasmas as usually shown in shell-like SNRs. Especially, we discovered a new feature of ionization state, "overionized", from IC 443 and W49B in a series of the analysis. Temperature gradients are also detected from the five remnants, in such a way that the plasma is cooler toward the limb. Moreover, we found the plasma structure of a hot interior surrounded by a denser cool shell from IC 443 and W49B. These plasma structures are consistent with those of shell-like SNRs, suggesting that MM SNRs have experienced similar evolutions as shell-like SNRs. In addition to above results, we revealed that thermal conduction can cause the hot interior plasma to be overionized by reducing temperature and density gradients within the remnants, leading to raise the density of the interior plasma. Therefore, we suggest that "center-filled" X-ray morphology is created as a result of thermal conduction, which should arise in all SNRs. This is consistent with the results that MM SNRs are in collisional ionization equilibrium since the conduction timescale is roughly similar to the ionization timescale. Hence, we reached the conclusion that MM SNRs are those evolved over ~104 yr, which we propose to call this phase as "conduction phase".

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