Nonlinear Gravitational-Wave Memory From Merging Binary Black Holes

Gravitational waves are oscillations in spacetime that propagate throughout the universe at the speed of light. They are a prediction of Einstein’s theory of General Relativity. Detectable sources of gravitational waves are typically collisions of black holes or other compact objects (neutron stars, white dwarfs). While most gravitational-wave signals are expected to be oscillatory in nature, some will exhibit a phenomenon called gravitational-wave memory. This refers to a non-oscillatory component of the gravitational wave signal that can leave a permanent distortion (or “memory” ) in a gravitational-wave detector. The nonlinear memory effect is a type of memory signal that arises when gravitational waves themselves produce gravitational waves. Merging binary black holes create the strongest nonlinear memory. These memory signals are difficult to model using conventional numerical relativity simulations. To address this issue we use a semi-analytic procedure to construct the memory signal from the non-memory (oscillatory) pieces of the gravitational-wave field. We construct these memory signals using the output of several numerical simulations of non-spinning, quasi-circular black hole binaries with varying mass ratios. Our results could be used to improve the detectability and interpretation of the memory effect by ground or space-based gravitational-wave detectors