A hybrid model for comparable-mass binary-black-hole gravitational waveforms

Gravitational waves (GW) from binary-black-hole mergers have three parts: inspiral, merger and ringdown. Post-Newtonian (PN) and black-hole-perturbation (BHP) theories model the inspiral and ringdown parts of the waveform, respectively. A hybrid approximation method was previously used during the merger by applying PN and BHP theories at the same times in different spatial regions and matching the results at a boundary where the theories were either both valid or had errors that did not affect the waveform. The prior work used leading PN dynamics and non-rotating BHP theory; this led to errors during the late inspiral and disagreement with the dominant quasinormal mode (QNM) frequency observed in numerical relativity (NR) simulations during the ringdown, respectively. We modified the hybrid method to achieve a better match with NR waveforms as follows: During the inspiral we used Effective-One-Body (EOB) dynamics rather than PN, and we evolved the waveform on an effective BHP theory using a modified Poschl-Teller potential tuned to match the dominant QNM of the remnant black hole. By optimizing the potential, the matching region, and adding an effective 1PN parameter, we could match NR waveforms for comparable mass ratios up to q = 8 with the accuracy of order 10^-3.