Metal-Rich Plasma at the Center Portion of the Cygnus Loop

We observed the center portion of the Cygnus Loop supernova remnant with the ASCA observatory. The X-ray spectrum of the center portion was significantly different from that obtained at the North-East (NE) limb. The emission lines from Si and S were quite strong while those of O and the continuum emission were similar to those obtained at the NE limb. Based on the spectral analysis, Si and S emission lines originated from a high-kTe and low ionization plasma whereas O and most of the continuum emission arose from a low-kTe and high ionization plasma. We suppose that Si and S emitting gas are present at the interior of the Loop while O lines and continuum emission mainly arise from the shell region. Therefore, we subtracted the spectrum of the NE limb from that of the center. Obtained abundances of Si, S, and Fe were 4 $\pm$ 1, 6 $\pm$ 2, and ${1.3}^{+0.6}_{-0.3}$ times higher than those of the cosmic abundances, respectively, and are $\sim$40 times richer than those obtained at the NE limb. These facts strongly support that some of the crude ejecta must be left at the center portion of the Cygnus Loop. The low abundance of Fe relative to Si and S suggests a type II SN with a massive progenitor star as the origin of the Cygnus Loop.