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Abstract

Observation and characterisation of gravitational waves from binary black
holes requires accurate knowledge of the expected waveforms. The late inspiral
and merger phase of the waveform is obtained through direct numerical
integration of the full 3-dimensional Einstein equations. The Spectral Einstein
Code (SpEC) utilizes a multi-domain pseudo-spectral method tightly adapted to
the geometry of the black holes; it is computationally efficient and accurate,
but--for high mass-ratios and large spins--sometimes requires manual
fine-tuning for the merger-phase of binaries. The Einstein Toolkit (ET) employs
finite difference methods and the moving puncture technique; it is less
computationally efficient, but highly robust.
For some mergers with high mass ratio and large spins, the efficient
numerical algorithms used in SpEC have failed, whereas the simpler algorithms
used in the ET were successful. Given the urgent need of testing the accuracy
of waveform models currently used in LIGO and Virgo inference analyses for high
mass ratios and spins, we present here a synergistic approach to
numerical-relativity: We combine SpEC and ET waveforms into complete
inspiral-merger-ringdown waveforms, taking advantage of the computational
efficiency of the pseudo-spectral code during the inspiral, and the robustness
of the finite-difference code at the merger. We validate our method against a
case where complete waveforms from both codes are available, compute three new
hybrid numerical-relativity waveforms, and compare them with analytical
waveform models currently used in LIGO and Virgo science. All the waveforms and
the hybridization code are publicly available.