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I understand that accretion of normal matter into a super-massive black hole leads to x-ray emissions. Is the same effect expected to occur for dark matter accretion into a supermassive black hole? Are there any good articles (e.g. on arXiv) I could read to learn more about this?

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    $\begingroup$ Normal matter interacts strongly with EM waves (like X-rays) but dark matter does not (assuming dark matter is the usual WIMP). In principle dark matter could emit X-rays, but you'd have to get it a lot hotter than normal matter. $\endgroup$ – John Rennie Jan 19 '14 at 11:46
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    $\begingroup$ In addition to the John's comments, the dark matter is not going to participate in the mechanics of accretion disk formation in the way that ordinary matter does, and the origin of the x-rays lies in the interaction of the disk with itself and with the magnetic field of the whole. $\endgroup$ – dmckee Jan 19 '14 at 15:31
  • $\begingroup$ @dmckee Why does "the dark matter [...] not [...] participate in the mechanics of accretion disk formation in the way that ordinary matter does"? Won't dark matter be "compressed" when it gets closer to the black hole, and then show similar effects than ordinary matter? $\endgroup$ – jochen Jan 19 '14 at 17:01
  • $\begingroup$ The accretion disk forms because the infalling dust and gas interacts with itself while still conserving angular momentum about the black hole. Look up Virial heating. The dark matter nearly does not interact, so instead of forming an accretion disk the bits of dark matter just keep orbiting in an unstructured ball. Note that dark matter is not completely non-interacting, so this statement is not absolute. $\endgroup$ – dmckee Jan 19 '14 at 17:31
  • $\begingroup$ Thanks all. If you write your answers as a formal "answer", I can formally "accept" it. $\endgroup$ – jochen Jan 21 '14 at 23:17
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Assuming dark matter is a WIMP, NASA made a computer simulation on dark matter particle annihilation to see what would happen when dark matter fell into a black hole. According to the article, it may produce gamma ray as the collisions of dark matter particles increase closer to the black hole.

Here is the preprint for the NASA simulation: http://arxiv.org/abs/1506.06728

And another one: http://arxiv.org/abs/0909.0169

There is, however, some controversy about the validity of these studies. See e.g. this article and this comment and references therein.

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