Tracking the precession of compact binaries from their gravitational-wave signal

We present a simple method to track the precession of a black-hole-binary system during the inspiral, using only information from the gravitational-wave (GW) signal. Our method consists of locating the frame from which the magnitudes of the (l=2, |m|=2) modes are maximized, which we denote the 'quadrupole-aligned' frame. We demonstrate the efficacy of this method when applied to waveforms from numerical simulations. In the test case of an equal-mass nonspinning binary, our method locates the direction of the orbital angular momentum to within (Δθ,Δφ)=(0.05 deg., 0.2 deg.). We then apply the method to a q=M2/M1=3 binary that exhibits significant precession. In general, a spinning binary's orbital angular momentum L is not orthogonal to the orbital plane. Evidence that our method locates the direction of L rather than the normal of the orbital plane is provided by comparison with post-Newtonian results. Also, we observe that it accurately reproduces similar higher-mode amplitudes to a comparable non-precessing binary, and that the frequency of the (l=2, |m|=2) modes is consistent with the ''total frequency'' of the binary's motion. The simple form of the quadrupole-aligned waveform may be useful in attempts to analytically model the inspiral-merger-ringdown signal of precessing binaries, and in standardizing the representation of waveforms for studies of accuracy and consistency of source modelling efforts, both numerical and analytical.