An X-ray Study of the Galactic Mixed-Morphology Supernova Remnants HB21 and CTB 1 using ASCA and Chandra

The morphological class of mixed-morphology supernova remnants (MM SNRs) are characterized by a shell-like radio morphology combined with a center-filled X-ray morphology with a thermal spectrum. Several competing evolutionary scenarios have been proposed to explain the origin of the unique combination of X-ray and radio morphologies of MM SNRs, but a comprehensive understanding of these sources has yet to be developed. We have conducted spatial and spectral analyses of the X-ray emission of several MM SNRs and to illustrate this work we will present our results for two known MM SNRs, HB21 (G89.0+4.7) and CTB 1 (G116.9+0.2). In the case of HB21, the X-ray emission is from a central plasma located entirely within the radio shell: the X-ray spectrum of this plasma features noticeable Si and S lines and can be fit adequately with thermal models, indicating that the X-ray emitting gas is close to ionization equilibrium. No clear X-ray spectral variations are seen when comparing emission from the northwestern and southeastern portions of this plasma. In contrast to HB21, the X-ray emission from CTB 1 extends through a gap in the radio shell of the SNR: the spectrum of this emission features lines associated with Mg and Si and is harder than the emission from HB21. This result indicates that either the X-ray emitting gas associated with this SNR has not yet attained thermal equilibrium or this emission has multiple thermal components. We have incorporated archived Chandra observations of CTB1 as part of this study: with the ASCA data, we have also discovered a hard point source associated with CTB 1 which features marginal ( ∼4σ ) evidence for pulsation with a period of 48 msec. Finally, we compare X-ray properties of HB21 and CTB 1 with other MM SNRs as published in the literature.