Mergers of black-hole binaries with aligned spins: Waveform characteristics

We conduct a descriptive analysis of the multipolar structure of gravitational-radiation waveforms from equal-mass aligned-spin mergers, following an approach first presented in the complementary context of nonspinning black holes of varying mass ratio [J.G. Baker et al. Phys. Rev. D 78 044046 (2008)]. We find that, as with the nonspinning mergers, the dominant waveform mode phases evolve together in lock-step through inspiral and merger, supporting the previous waveform description in terms of an adiabatically rigid rotator driving gravitational-wave emission-an implicit rotating source. We further apply the late-time merger-ringdown model for the rotational frequency introduced in [J.G. Baker et al. Phys. Rev. D 78 044046 (2008)], along with an improved amplitude model appropriate for the dominant (2, \pm2) modes. This provides a quantitative description of the merger-ringdown waveforms, and suggests that the major features of these waveforms can be described with reference only to the intrinsic parameters associated with the state of the final black hole formed in the merger. We provide an explicit model for the merger-ringdown radiation, and demonstrate that this model agrees to fitting factors better than 95% with the original numerical waveforms for system masses above \sim150M\odot. This model may be directly applicable to gravitational-wave detection of intermediate-mass black-hole mergers.