What characteristic of a supernova remnant causes it to glow intensely in X-rays?

Answer

Gas heated by the shock wave to millions of degrees

The shock wave heats the gas it sweeps up to millions of degrees Celsius, causing this extremely hot material to become a powerful emitter of X-rays before it cools sufficiently.

What characteristic of a supernova remnant causes it to glow intensely in X-rays?
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