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Are there any known binary systems in which one member is a black hole and the other is not a black hole (main sequence star, giant, neutron star, white dwarf, whatever)?

Googling seems to turn up only discussion of binaries in which both components are black holes. Is there some astrophysical reason why the kind of system I'm talking about cannot form? I think I remember that gravitational wave observatories have talked about black hole-neutron star mergers, but are such systems purely hypothetical so far?

I guess Sagittarius A* is sort of close to being an example, if you count it and the stars orbiting it (or orbiting their common barycenter) as a multiple star system.

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  • $\begingroup$ Google is only showing hits for black-hole pairs because that's where the exciting research (gravity waves) is taking place right now. $\endgroup$
    – Mark
    Oct 3, 2017 at 17:16
  • $\begingroup$ @Mark I can detect Gravity waves at the beach. Perhaps you mean gravitational waves? $\endgroup$
    – ProfRob
    Oct 3, 2017 at 22:49
  • $\begingroup$ I made that post over a year ago and was cleaning all that up. I got suspended today for trying to improve my questions. You can fix that black whole question if you want but I no longer can make edits there. $\endgroup$
    – Muze
    Feb 12, 2019 at 23:02
  • $\begingroup$ different but related: Binary pairings that haven't been discovered yet? $\endgroup$
    – uhoh
    Jun 29, 2021 at 15:15
  • $\begingroup$ LIGO/Virgo literally just published a paper about two BH-NS mergers yesterday! iopscience.iop.org/article/10.3847/2041-8213/ac082e $\endgroup$
    – Daddy Kropotkin
    Jun 30, 2021 at 14:58

3 Answers 3

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Black hole and main sequence star/giant star

We can observe binary systems containing a black hole by looking for emissions from accretion disks which may form when matter is transferred from the companion star. X-ray binaries and microquasars are particularly notable types.

The compact object doesn't have to be a black hole - neutron stars also sometimes show up - but in some cases, measurements indicate that the remnant is a black hole. In other cases, either the mass of the compact object is poorly constrained or could still be a massive neutron star, so for many systems, the nature of this object remains unknown.

Examples:

  • Cygnus X-1: The companion star here is HDE 226868, a massive O-type star.
  • V404 Cygni: The companion star is a K-type star (much less massive than HDE 226868); the system forms a microquasar, with occasional high-energy x-ray emission.

Black hole and white dwarf

It seems a logical assumption that black hole/white dwarf systems should be possible, and indeed they are. Mass loss through accretion by a companion object can change a star's evolutionary future (see the Algol paradox for a particularly weird case), but for many systems with a black hole, the "normal" star should still progress through the main sequence and post-main-sequence evolution, possibly becoming a white dwarf.

Matter transfer can still happen at this stage, of course, so emission may continue, although the nature of the radiation could be changed. It's true that there aren't many known systems of this type, but there almost certainly more.

Examples:

  • X9: The companion star is a white dwarf orbiting extremely close to the black hole. Bahramian et al. are confident that the companion is a white dwarf, and the main object is likely a black hole.

Black hole and neutron star

We now have two reasonable candidate neutron star-black hole systems thanks to a pair of LIGO detections observed in January 2020 and recently released. The primary components of both systems are certainly black holes, while the secondary components have masses that are fairly consistent with our understanding of neutron star mass limits. It's certainly possible that one or both could be an exotic compact object or a primordial black hole, but a neutron star nature seems far more likely.

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    $\begingroup$ Is the failure to detect a BH-NS system yet really puzzling? The abstract on the article you linked says that they were only able to constrain the merger rate enough to eliminate the most optimistic models, and that it would take several more observing runs with no results before a continued failure became problematic. $\endgroup$
    – Dan Is Fiddling By Firelight
    Oct 3, 2017 at 13:43
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    $\begingroup$ @DanIsFiddlingByFirelight LIGO/Virgo just released yesterday a new paper about two BH-NS detections iopscience.iop.org/article/10.3847/2041-8213/ac082e $\endgroup$
    – Daddy Kropotkin
    Jun 30, 2021 at 14:59
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In general those binaries will be hard to detect, but there is no reason why they wouldn't form.
A whole class of objects, X-Ray binaries are in fact thought to be hosting a pair of BH and non-BH objects. The idea here is that the X-ray luminosity originates from mass accreted from the companion through Roche-Lobe-overflow.
The mass from the companion or 'donor' falls down the gravitational well of the BH, rams into its accretion disc and creates Bremsstrahlung as it deaccelerates violently. This radiation falls into the wavelength of X-Rays and escapes the binary system, to be picked up by our telescopes later.

In a small number of cases we've been successful in determining what the binary companion is, depicted in this famous graphic by Orosz:

enter image description here

See also here for more information

This graphic depicts the binary distance, approximate size of the accretion disc, and size of the companion star. The companion stars are very huge, because they reached their red giant stage, not because they have some other anomalous properties.

This stage is a normal ending of a star's life, just like what our sun will experience. Just in this case there's this black hole in the vicinity that has quite strong tides, which are able to strip away parts of the evolved star's outer shell.

This graphic is a few years old now, so I'm not sure what the recent state of affairs concerning the total number of those objects is. Also there are many more unconfirmed BH binary candidates amongst the X-ray binary population, and I don't even know anything about BH-exotic things pairings. But maybe someone else can comment on that.

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Prior to the discovery of gravitational wave sources, the only confirmed stellar-sized black holes, were those detected in binary systems with more "normal" stars.

These were most detected via X-rays emitted by hot material that is accreted from the "normal" companion. The two main categories are high-mass and low-mass X-ray binaries: in the former, the black hole accreted the wind from a massive stellar companion; in the latter, the mass is transferred by Roche lobe overflow.

The black hole status is confirmed by measuring a dynamical mass for the unseen companion that exceeds any plausible upper limit for a stable neutron star. Read the review by McClintock (2003).

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