Detection of the Permanent Strain Offset Component of Gravitational-Wave Memory in Black Hole Mergers

This paper reports direct measurements of the permanent space-time strain offset component of the gravitational-wave memory predicted by general relativity to accompany black hole mergers. Such a change in strain, persisting after the wave itself is long past – measurable in principle but with considerable difficulty – has heretofore eluded detection. Since it is independent of the form of the time-development of the memory strain, this approach circumvents the need for precise modeling of the memory and non-memory signals. A model-independent analysis using a template-like algorithm, applied to a selection of 67 observations of 41 black hole merger events in the LIGO/Virgo Gravitational Wave Transient Catalog, yields a mixture of probable detections and upper limits. The associated statistical significance is assessed using parallel analysis of a large number of intervals shifted in time away from the mergers. The resulting p-value for a reasonably formulated null hypothesis (no real signals anywhere in the ensemble) is less than .003. Appendices contain MatLab code for the operations in the analysis, including an algorithm for the complex Fourier transform for arbitrarily spaced data.