Time- and frequency-domain waveform models for the gravitational-wave memory effect

The nonlinear gravitational-wave (GW) memory effect is a strong-field gravitational phenomenon that arises from interactions between the GWs with themselves. This effect manifests as a lasting offset in the GW strain that persists after the passage of a GW. Although the detection of the memory effect is challenging because the signal is relatively weak (compared to the dominant quadrupolar GWs), it is possible to measure the memory signal in a population of binary-black-hole (BBH) mergers with the current GW detectors or from individual BBHs with next-generation ground- and space-based GW detectors. Template-based searches for the memory effect require evaluating the waveforms many times, and they can benefit from fast-to-evaluate and accurate waveform models. We present a stand-alone time-domain waveform model for the GW memory effect for nonspinning BBH mergers, which accurately models just the GW signal associated with the memory effect and not other linear or nonlinear GW phenomena. The time-domain model incorporates information from post-Newtonian theory and black-hole perturbation theory. We find its typical mismatch in LIGO is of order $10^{-3}$. We also present an independent phenomenological frequency-domain model for the GW memory effect.