Gravitational Wave Ringdown

After black hole merger, the remnant oscillates at quasinormal mode (QNM) frequencies. The ringdown is a superposition of damped sinusoids: h ~ Σ A_lmn e^−t/τ cos(ω_lmnt).

QNM frequencies depend only on M, a, Q (no-hair theorem). GW150914 ringdown ~ 250 Hz.

BH Mass M/M☉ 30 M☉ Spin a/M 0.70 Modes 2 Noise σ 0.05