ASCA Observations of the Supernova Remnant IC 443: Thermal Structure and Detection of Overionized Plasma

We present the results of X-ray spatial and spectral studies of the "mixed-morphology" supernova remnant IC 443 using ASCA. IC 443 has a center-filled image in the X-ray band, contrasting with the shell-like appearance in radio and optical bands. The overall X-ray emission is thermal, not from a synchrotron nebula. We observed IC 443 three times with ASCA, covering the whole remnant. From the image analysis, we find that the softness-ratio map reveals a shell-like structure. At the same time, its spectra require two (1.0 and 0.2 keV) plasma components; the emission of the 0.2 keV plasma is stronger in the region near the shell than in the center. These results can be explained by a simple model that IC 443 has a hot (1.0 keV) interior surrounded by a cool (0.2 keV) outer shell. From the emission measures, we infer that the 0.2 keV plasma is denser than the 1.0 keV plasma, suggesting pressure equilibrium between the two. In addition, we find that the ionization temperature of sulfur, obtained from the H-like to He-like Kα intensity ratio, is 1.5 keV, significantly higher than the gas temperature of 1.0 keV suggested from the continuum spectrum. The same can be concluded for silicon. Neither an additional, hotter plasma component nor a multitemperature plasma successfully account for this ratio, and we conclude that the 1.0 keV plasma is overionized. This is the first time that overionized gas has been detected in a supernova remnant (SNR). For the gas to become overionized in the absence of a photoionizing flux, it must cool faster than the ions recombine. Thermal conduction from the 1.0 keV plasma to the 0.2 keV plasma could cause the 1.0 keV plasma to become overionized, which is plausible within an old (3 × 104 yr) SNR.