What material generates the hot gas responsible for the long-lasting thermal X-ray emission?

Answer

The shocked gas of the Interstellar Medium (ISM)

The X-ray glow is generated by the hot, shocked gas resulting from the blast wave slamming into the surrounding Interstellar Medium, which is distinct from the material powering the initial optical emission.

What material generates the hot gas responsible for the long-lasting thermal X-ray emission?
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