What structural feature often characterizes Mixed-Morphology (MM) supernova remnants when viewed in X-rays?

Answer

A centrally filled shell structure

Mixed-Morphology (MM) remnants are often characterized by exhibiting a centrally filled shell structure when observed in X-rays, implying interaction with a heterogeneous interstellar medium.

What structural feature often characterizes Mixed-Morphology (MM) supernova remnants when viewed in X-rays?
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