Extends the CCEGA gravitational wave echo prediction from Schwarzschild to Kerr black holes. The echo frequency spacing Δf decreases monotonically with spin: for a*=0. 99, Δf ≃ 0. 25 × ΔfSchwarzschild. The complete prediction is a two-dimensional surface Δf (M, a*), tabulated for M ∈ 5, 50 Msun and a* ∈ 0, 0. 99. A measurement of Δf by the Einstein Telescope, combined with the chirp mass M, uniquely determines the remnant spin a* — providing a spin measurement independent of ringdown analysis. This transforms the CCEGA echo prediction from a binary detection into a quantitative measurement: CCEGA becomes a spin measurement tool. The Δf (M, a*) surface is a unique, zero-free-parameter prediction with no GR analogue.
Marc López Sánchez (Mon,) studied this question.