Waveform models for the gravitational-wave memory effect

The gravitational-wave (GW) memory effect causes a lasting relative displacement of freely falling observers before and after the passage of a transient source of GWs. The effect was first computed in the 1970s, but only with upcoming improvements to the LIGO, Virgo, and KAGRA detectors will the prospects of detecting the effect in a population of binary-black-hole (BBH) mergers be promising. Methods to detect the memory effect require accurate waveform models that can be evaluated rapidly, because these methods to assess the significance of the memory require many waveform evaluations. Current analytical waveform models, and many numerical-relativity (NR) simulations and surrogates, do not include the waveform related to the GW memory effect; instead, the waveform is computed as a secondary step from waveform models without the memory by employing conservation laws in asymptotically flat spacetimes. We instead develop stand-alone time- and frequency-domain waveform models of the GW memory effect for nonspinning BBH mergers. We incorporate data from both NR surrogate models and the extreme mass-ratio limit to develop a waveform model that can be applied to all mass ratios.