Nonlinear effects in the black hole ringdown: absorption-induced mode excitation

Understand

Gravitational-wave observations of black hole ringdowns are commonly used to characterize binary merger remnants and to test general relativity.

• These analyses assume linear black hole perturbation theory, in particular that the ringdown can be described in terms of quasinormal modes even for times approaching the merger.
• Here we investigate a nonlinear effect during the ringdown, namely how a mode excited at early times can excite additional modes as it is absorbed by the black hole.
• This is a third-order secular effect: the change in the black-hole mass causes a shift in the mode spectrum, so that the original mode is projected onto the new ones.