It has something to do with X-ray astronomy, so extrasolar planets are out.

Neutron stars were discovered in the 1960s, black holes belonged to the realm of speculation before high-energy astronomy so they both are probably out of question.

Cataclysmic variables involving white dwarfs? Hot interstellar gas?

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman