Its oval shell-like, nonthermal structure in the radio continuum supports the view that Sgr A East is a supernova remnant (SNR), although other explanations have also been discussed, e.g., multiple explosions based on the, originally derived total kinetic energy of 6 x 1052 erg . However, measurements with the Chandra Observatory and XMM-Newton have revealed that the total energy of the hot plasma is much smaller («1.5 x 1049 erg), a value even smaller than the nominal energy for a single SNR. Thus one SNR can easily account for Sgr A East . Since the plasma has already reached thermal equilibrium, the estimate does apply to the full thermal energy in the observable X-ray band. With an age of about 8 000 y , the mass of the plasma may originate as either the ejecta or swept-up interstellar material. The X-ray spectrum is enriched by heavy elements (see later), which suggests that the plasma is dominated by the supernova ejecta. SNRs showing centrally concentrated thermal X-rays lying within a shell-like nonthermal radio shell are defined as a relatively new class of SNRs and are called "mixed morphology" (MM) supernova remnants. About 20 members of this class are identified up to now, including Sgr A East itself .
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