Kerr ISCO
≈ 42.3%
Calculated
Radiative efficiency of a thin disk at the prograde ISCO of an extreme Kerr hole
An illustration of the Milky Way black hole. Not a diagram of the extreme Kerr case.
IllustrationNASA, ESA, CSA, Ralf Crawford (STScI)PUBLIC DOMAIN
_(2025-110).png){kind=link}
For a = M, specific energy at ISCO is 1/√3, so η = 1 − 1/√3 ≈ 42.3%. Physical spin is less than 1.
What it supports
A general-relativity identity in the test-particle Kerr metric. Neighbor digits to rainbow 42.3°, not the same object.
What it does not
Not a measured 42%. Not the same 42.3 as the primary bow.
Calculation
η = 1 − 1/√3 ≈ 0.4226 for a/M = 1 prograde
Kerr geodesic specific energy at the innermost stable circular orbit
ExactnessDerived mathematics
FormOther form
Unit%
Strengthhard