I was studying stellar mass black holes and wondered why almost all of them had a companion star, I came through many explanations but was not satisfied, is it because its hard to detect them without one cause there accretion disk wouldn't be visible or the orbital dynamics would be hard to infer.


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An isolated (stellar mass) black hole is not expected to produce electromagnetic signals of note. The only way we are likely (used in a very loose sense) to observe them is through micro lensing events (which would almost certainly be one-offs).

Consequently, most known (documented) stellar mass black holes have a companion, because this is what allows us to observe them in the first place.

  • $\begingroup$ Is it possible to even roughly estimate how many isolated black holes there are? $\endgroup$ Commented May 20 at 22:26
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    $\begingroup$ Black holes do not emit light, so we cannot observe them unless they are accreting matter. Isolated black holes do not accrete matter, so we cannot see them. If they merge, we might detect the gravitational waves, but this requires them to interact — which means they are no longer isolated. Then again, some have wondered if black holes are the source of dark matter. So maybe we could. $\endgroup$ Commented May 20 at 23:04
  • $\begingroup$ @GregBurghardt, we can also see isolated black holes through gravitational lensing events. $\endgroup$
    – Mark
    Commented May 20 at 23:21
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    $\begingroup$ @Peter-ReinstateMonica Yes. Stare at the same tiny patch of sky for a long period and count how many microlensing events are seen. Repeat for other patches to get a random sample. Extrapolate for the whole sky. Trouble is, that experiment is extremely resource intensive. See An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing. $\endgroup$
    – Schwern
    Commented May 20 at 23:35

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