Nonlinear Gravitational-wave Memory from Binary Black Hole Mergers

Some astrophysical sources of gravitational-waves can produce a “memory effect,” which causes a permanent displacement of the test masses in a freely-falling gravitational-wave detector. The Christodoulou memory is a particularly interesting nonlinear form of memory that arises from the gravitational-wave stress-energy tensor’s contribution to the distant gravitational-wave field. This nonlinear memory contributes a non-oscillatory component to the gravitational-wave signal at leading (Newtonian-quadrupole) order in the waveform amplitude. Previous computations of the memory and its detectability considered only the inspiral phase of binary black hole coalescence. Using an “effective-one-body” (EOB) approach calibrated to numerical relativity simulations, as well as a simple fully-analytic model, the Christodoulou memory is computed for the inspiral, merger, and ringdown. The memory will be very difficult to detect with ground-based interferometers, but is likely to be observable in supermassive black hole mergers with LISA out to redshifts z . 2. Detection of the nonlinear memory could serve as an experimental test of the ability of gravity to “gravitate.” Subject headings: black hole physics—gravitation—gravitational waves—relativity