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When I see images of accreting objects, the gas from the donor has a white flash where it meets the accretion disk, like in the image below. Why does this happen if it is true, and why is it there? Cygnus X-1

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    $\begingroup$ What's the source of the image? Can you credit the work? Thanks! Also, this may provide context for answering. Otherwise asking "Why does this artist's conceptual rendering look this way" might be seen as asking for people to guess about an artistic drawing. $\endgroup$
    – uhoh
    Feb 28 at 2:53
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When the stream of gas falls towards the disk it gains a significant fraction of the orbital kinetic energy (after all, it is falling from the top of the Roche lobe) which means that it is moving fast and then slows down sharply when it interacts with the disk. This produces a hot spot that in theory could reach $10^8$ K but in practice "merely" is very hot.

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