Ejecta detection in middle-aged large magellanic cloud supernova remnants 0548-70.4 and 0534-69.9

We have observed supernova remnants 0548-70.4 and 0534-69.9 in the Large Magellanic Cloud (LMC) with the Chandra X-Ray Observatory and report on the X-ray spectral analysis. Our images of 0548-70.4 and 0534-69.9 show bright central regions as well as brightened limbs. The X-ray spectra from the central regions exhibit enhanced metal abundances, in significant contrast to limb spectra, which show abundances consistent with the LMC interstellar medium (ISM). Considering the relatively old ages (similar to10,000 yr), these supernova remnants might be assumed to be in the Sedov phase, in which the X-ray spectra would be dominated by swept-up ISM material. The detection of high abundances in these old remnants is therefore surprising. Spectra from the limb regions were analyzed with Sedov models. The results were then used to account for blast wave emission seen in projection toward the central region and were added to a plane-parallel shock model for the reverse shock in the ejecta. We find elevated levels of iron, oxygen, magnesium, silicon, and sulfur in the bright central regions of each remnant. We introduce a new X-ray spectral shock model appropriate for heavy-element-dominated plasmas, in which electrons liberated by successive ionizations dominate the electron pool and modify the electron temperature pro. le. With this model, we find reverse-shock speeds of 420 km s(-1) for 0548-70.4, 500 km s(-1) for the northeast central region of 0534-69.9, and 360 km s(-1) for its south central region. The elemental abundances favor a Type Ia supernova origin for both 0548-70.4 and 0534-69.9.