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Although the circular photon orbit and ISCO for the Kerr black hole remain distinct from each other and from the horizon in the extremal spin limit on a constant Boyer-Lindquist time slice, on a horizon-crossing slice they both coincide with the null generators of the horizon.
J. M. Bardeen, W. H. Press, S. A. Teukolsky, “Rotating black holes: Locally nonrotating frames, energy extraction, and scalar synchrotron radiation,” Astrophys. J. 178
1972
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C. Doran, “A New form of the Kerr solution,” Phys. Rev. D61
2000
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