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Rotating black holes are formed due to the gravitational collapse of massive spinning objects. And, it is generally believed that Kerr black hole solutions are valid for the empty space outside of the astrophysical black holes.

My question: Are there some pieces of evidence (observational data) for astrophysical black holes indicating deviation from Kerr black hole solution?

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    $\begingroup$ Not quite an observation, but arxiv.org/pdf/1904.04654.pdf suggests that ultra-high energy cosmic rays could be the result of interactions with charged black holes. These would deviate from the Kerr solution, to the Kerr-Newman solution. I might be able to work that into an answer if that is the sort of thing you are looking for.. $\endgroup$ – James K Mar 25 at 20:17
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    $\begingroup$ @JamesK, Thanks for your valuable comment. I read the abstract of that paper and it seems really interesting (in fact, I'm a fan of testing this idea, i.e., the possibility of a small charge in some black holes). FYI, I'm looking for any observational deviation from Kerr black holes which can be explained using a broader version of rotating black hole solution such as Kerr-Newman black holes or non-trivial rotating black holes in modified gravity. $\endgroup$ – Ad Astra Mar 25 at 20:43
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Evidence of "new physics" seems to be lacking (in my research into this question.) Black holes can be described fully by known consequences of general relativity as far as our ability to observe them. There are known deviations from the Scharwchild metric, for example in black holes in which the accretion disc is inclined relative to the spin of the black hole, and in which the effects of frame-dragging can be observed as the accretion disc transtions to the spin plane of the black hole.

High energy cosmic rays are a poorly understood phenonomum. They seem to be charged particles (ie protons) that have been acclerated to very high velocities (much higher than 99% c) by black holes. But the particular mechansim that does this is unclear. A paper, https://arxiv.org/pdf/1904.04654.pdf, proposes that high energy protons may be an consequence of black holes with a Kerr-Newmann metric, that is a rotating charged black hole; as even a small charge would have a singificant effect on protons in the neighbourhood. This could be interpreted as evidence that some black holes have a Kerr-Newman spacetime metric.

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  • $\begingroup$ “ There are known deviations from the Scharwchild [sic] metric, for example in black holes in which the accretion disc is inclined relative to the spin of the black hole,” If the black hole has spin, it wouldn’t be Schwarzschild in the first place. Did you mean “deviations from Kerr”? $\endgroup$ – mmeent Mar 26 at 8:24
  • $\begingroup$ @JamesK, Thanks for paying attention to my question. Your answer is interesting, and I consider it in my future work; so I've upvoted. That paper (arxiv.org/pdf/1904.04654.pdf) is still a proposal rather than an evidence. I'd argued that "a small charge" that authors assumed is not small as they claimed (I think 1 Coulomb per solar mass is not small). However, it's believed any net charge of a black hole will quickly attract the opposite charge which leads to neutralizing charged black hole, but I agree that the charge parameter in black holes should not be neglected a priori. $\endgroup$ – Ad Astra Mar 26 at 21:43
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    $\begingroup$ @mmeent No, I mean there are directly observed deviations from the schwatzchild metric. But there are no directly observed deviation from the kerr metric, unless you count the suggestion that ultra high energy cosmic rays are evidence of kerr-newcome mettrics. $\endgroup$ – James K Mar 26 at 22:21

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