We characterize the phase transition at Mcrit = 14. 75 M☉ from the perspective of GW echo observability. The transition is first-order in echo amplitude: hₑcho = hₘax for M Mcrit (CCEGA BH, horizon). The echo timescale is continuous through Mcrit, reaching tₑchoᶜrit ≈ 0. 100 ms, then jumping to tₑchoBH ≈ 0. 195 ms just above Mcrit as the echo source switches from the exterior surface to the interior surface inside the horizon. A population of mergers spanning Mcrit would show a bimodal distribution in echo timescales with a gap between ~0. 10 ms and ~0. 19 ms — a unique CCEGA prediction testable with Einstein Telescope.
Marc López Sánchez (Sun,) studied this question.