So as far as I know, the objects that have been confirmed to be black holes by direct observation of the event horizon("black hole shadow"), like M87 for instance, also show observable evidence of frame dragging (the characteristic of a black hole with a Kerr metric). Now the thing about Schwarzschild black holes is that a point singularity of infinitesimal size and infinite density is a direct violation of the uncertainty principle. Cygnus X-1 is often listed as a Schwarzschild black hole, but is there any observational evidence that is has both an event horizon and lack of frame dragging?
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7$\begingroup$ No black holes are Schwarzschild black holes. Everything spins to some extent. $\endgroup$– ProfRobCommented May 30, 2020 at 22:31
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1$\begingroup$ See the graph at the end of this answer astronomy.stackexchange.com/a/20292/16685 which shows the spin of 19 supermassive black holes. I expect that stellar black holes are also have high spin parameters. $\endgroup$– PM 2RingCommented May 30, 2020 at 22:53
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2$\begingroup$ Also note that, strictly speaking, a pure Schwarzschild BH is eternal, and it's in a universe that doesn't contain anything else. But the Schwarzschild solution is still a useful approximation. $\endgroup$– PM 2RingCommented May 30, 2020 at 22:57
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1$\begingroup$ @StephenG Fair point, which is why I just said I expect them to have high spin, but I can't find much in the way of solid observational data about stellar mass BH spin. I assume they'd inherit a lot of angular momentum from their progenitor star. OTOH, supernova explosions tend to be quite asymmetrical, so I suppose that affects the remnant's angular momentum as well as its linear momentum. $\endgroup$– PM 2RingCommented May 31, 2020 at 0:44
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2$\begingroup$ The Cyg X-1 object is an accreting black hole in a binary system. It cannot have zero spin. $\endgroup$– ProfRobCommented May 31, 2020 at 8:29
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2 Answers
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All the available evidence suggests that the known compact objects in binary systems (high-mass and low-mass versions) have high spin parameters in general (e.g. Nielsen 2016. They have ample opportunity to spin up during their progenitor phase, through tidal locking, the supernova explosion and then by accreting material from a companion afterwards (which is how they are detected and how their spins are measured). In particular, Cygnus X-1 is thought to be a "near-extreme" (Nielsen 2016) example of a black hole with an "extreme value of spin" of $a>0.95$ (Gou et al. 2011).
The available evidence from merging black hole binaries is that the spins inferred from gravitational wave signatures are not extremal but low and poorly constrained, but could be consistent with zero in some cases (e.g. Tiwari et al. 2018).
The black holes observed in the centre of M87 and the centre of the Milky Way have almost circular "shadows" that could be consistent with Schwarzschild black holes. Equally, they could be consistent with Kerr black holes Bambi et al. 2019). On the other hand a complex analysis of the light ring (that I do not claim to understand) has suggested a spin parameter of $0.8 < a < 1.0$ for the M87 black hole (Tamburini et al. 2020). Daly et al. (2023) use the dynamics of gas close to the Milky Way black hole to infer a spin parameter of $0.90\pm 0.06$.
The idea of a non-spinning Schwarzschild black hole is an idealisation that seems unlikely to be exactly met in nature, though it might serve as an approximation for some rare, low-spinning high-mass black hole binary systems. Of course we know very little about isolated black holes, although we do know that newly born neutron stars spin rapidly...
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$\begingroup$ A spin of 0.95 would not be “near extremal”. Typical near extremal behavior does not set in until well above 0.99 or even 0.999. $\endgroup$– TimRiasCommented May 31, 2020 at 23:36
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$\begingroup$ @RobJeffries Thanks for an actual answer and not just a comment. I did wonder about this since a point singularity would pose a huge problem for physics as we understand it. $\endgroup$– Mr XCommented Jun 3, 2020 at 3:16
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We haven't even observed a nonspinning star or planet, and since there are much more of those than black holes the likelyhood for a nonspinning black hole is even smaller. You could create one artificially though by finetuningly feeding it with retrograde material, but the chances for that to happen in nature are almost zero.
