Quasinormal-mode filters: A new approach to analyze the gravitational-wave ringdown of binary black-hole mergers

We propose two frequency-domain filters to analyze ringdown signals of binary black hole mergers. The first rational filter is constructed based on a set (arbitrary) quasinormal modes (QNMs) the remnant holes, whereas second full comes from transmissivity holes. can remove corresponding QNMs original time-domain ringdowns, while changing early inspiral in trivial way---merely time and phase shift. After filtering out dominant we visualize existence various subdominant effects. For example, by applying our GW150914-like numerical relativity (NR) waveform, find second-order effects $(l=4,m=4),(l=5,m=4)$, $(l=5,m=5)$ harmonics; spherical-spheroidal mixing mode $(l=2,m=2)$ harmonic; $(l=2,m=1)$ harmonic due gravitational recoil. In another NR simulation where component spins are antialigned with orbital angular momentum also retrograde modes. sensitive properties (i.e., mass spin) thus have potential application future data analyses parameter estimations. investigate stability filter. Its connection instability QNM spectra discussed.