Quasinormal mode

Summary

A quasinormal mode is a damped oscillation of an open system, such as a perturbed black hole, that loses energy by radiating it away. It has a complex frequency: the real part gives the oscillation and the imaginary part the decay rate. After two black holes merge, the remnant rings down in a sum of quasinormal modes, which carry gravitational waves to distant detectors. For a Kerr black hole the frequencies depend only on the mass and the spin.