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Jeonghee Rho (IPAC/CalTech)
While shell-like, Crab-like, and composite supernova remnants (SNRs) represent well-defined classes, another group of SNRs with center-filled X-ray and shell-like radio morphology, forms an unique category of ``mixed morphology" (MM) SNR because of distinct physical and environmental properties (Rho & Petre 1998, ApJL, 503, 167).
We present ASCA observations of two candidate MM supernova remnants HB 21 and Kes 27. Two pointings (north and east) were observed to cover most bright part of HB21 and one pointing to cover Kes 27. The exposure time of HB21 for good data is 35-40 ks for each pointing, and that of Kes 27 is 20 ks. The ASCA images for both remnants confirm that the X-ray morphology is centrally concentrated within the well-defined radio shell. The emission is smoothly distributed in the interior, and additionally two X-ray bright peaks appear in Kes 27. The ASCA spectra for both remnants reveal that the emission is primarily thermal, HB21 with strong Si and S lines, and weak Fe L, Mg and Ar lines, and Kes 27 with strong lines of Mg, Si, S, and Fe K. The HB 21 spectra fitting yields an acceptable fit with a thermal (Raymond-Smith or Mekal) model with a line of sight absorption of NH~1.5\times1021 cm-2, and a temperature kT~0.68 keV, and no emission above 6 keV was detected from HB21. The temperature distribution of HB 21 is uniform, similar to those of the archetypes of MM SNR (MSH 11-61A and W44). In contrast, the Kes 27 spectra show a hard energy tail (between 5 to 10 keV). Although the Galactic plane region toward Kes 27 has complicated Galactic Ridge emission (GRE), the hard energy maps prove that the hard emission is indeed from the remnant itself. The hard emission peak is located between the two X-ray bright peaks, implying that there is spectral variation across Kes 27. The spectral distribution is different between the two remnants despite of similarity of center-filled X-ray morphology. I will discuss the ISM environments of the remnants using CO maps, and discuss implication of the ASCA results in MM SNR evolution.
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