Search for black hole ringdown signals in LIGO S4 data

The final state of a compact binary coalescence is expected to be a perturbed black hole which will radiate away the perturbations as gravitational waves. At late times, the distorted black hole will be sufficiently similar to a Kerr black hole [1] that black hole perturbation theory can be used to model the waveforms [2]. These ringdown waveforms are a superposition of quasi-normal modes, the dominant of which is expected to be a spherical harmonic of spin weight 2. The frequency and damping time uniquely depend on the mass and spin of the black hole. This uniqueness is directly related to the no-hair theorem and a reliable detection of ringdown waveforms would provide a key test of general relativity in the strong field regime [3].