Modeling ringdown. II. Aligned-spin binary black holes, implications for data analysis and fundamental theory

The aftermath of binary black hole coalescence is a perturbed remnant whose gravitational radiation rings down, encoding information about the new black hole's recent history and current state. It is expected that this ringdown radiation will be composed primarily of Kerr quasinormal modes, and thereby enable tests of general relativity. Here, the first complete ringdown signal model for nonprecessing binary black hole systems is presented: multipole amplitudes and phases are modeled as functions of initial binary parameters. It is found that using the peak time of the dominant merger multipole as a reference results in the dominant mode's excitation being a remarkably simple linear function of system parameters, strongly suggesting that an analytic treatment may be within reach. In particular, for initially nonspinning black holes, the dominant quadrupole is excited as -4 times the system's symmetric mass ratio. Application of the model to parameter estimation allows general relativity predictions for mode amplitudes independently of signal strength. Treatment of GW150914 indicates some mode amplitudes and relative phases are intrinsically difficult to constrain.