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Paper: Discovery of Overionized Plasma in the Mixed-Morphology SNR IC 443
Volume: 289, The Proceedings of the IAU 8th Asian-Pacific Regional Meeting, Volume I
Page: 207
Authors: Kawasaki, M. T.; Ozaki, M.; Nagase, F.
Abstract: We discovered overionized plasma in IC 443, which is one of the ``Mixed-Morphology'' supernova remnants (MM SNRs). Using ASCA data, we found that IC 443 has a plasma structure of an inner-hot (1.0 keV) and an outer-cool (0.2 keV) region. In addition, we found that the silicon and sulfur H-like line emission is too strong, considering the gas temperature about 1.0 keV suggested from the continuum shape of the spectrum. The intensity ratio of H-like Kalpha to He-like Kalpha requires a temperature about 1.5 keV, which is more highly ionized than what would be expected if the plasma were in 1.0 keV collisional equilibrium. We therefore conclude that the 1.0 keV plasma is overionized, which is the first such detection in SNRs. To make the plasma overionized, the gas should cool faster than the ions recombine to their equilibrium level, and thermal conduction from the hot interior to the cool outer shell could cause the interior plasma to become overionized. Among other MM SNRs, W44 shows no evidence of overionization while Kes~27 shows an implication of it, which suggests that not all of MM SNRs have overionized plasma.
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