If the mass and orbital period for two neutron stars is known before collapse to a black hole and the rate of spin is known after collapse, 100 revolutions per second for instance, that will correlate mathmatically to a specific radius.

Does this radius correlate to a physical feature of the black hole such as a surface? Does this radius fall between the radius of the physical substance and the Schwarzschild radius? Is it the largest, intermediate, or the smallest of these?

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    $\begingroup$ Black holes are characterised by an angular momentum, not an angular velocity (which is unknown/undefined). $\endgroup$ – Rob Jeffries Sep 18 '19 at 21:46
  • $\begingroup$ A black hole doesn't have a physical surface, but it has one or more horizons. See en.wikipedia.org/wiki/Rotating_black_hole $\endgroup$ – PM 2Ring Sep 19 '19 at 10:38
  • $\begingroup$ @RobJeffries There is an angular velocity associated to a black hole $\omega_{H} =\frac{a}{2M r_{+}}$, which is often interpreted as the angular velocity of the horizon. Specifying $M$ and $\omega_H$ is of course full equivalent to specifying $M$ and $J$ or $M$ and $a$ $\endgroup$ – mmeent Sep 20 '19 at 7:51

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