Probing soft signals of gravitational-wave memory with space-based interferometers

TL;DR

This paper explores the potential for future space-based interferometers to detect soft gravitational-wave memory signals, which are low-frequency, non-oscillatory signals predicted by general relativity.

Contribution

It demonstrates that soft displacement-memory signals can be detected and measured with high confidence using space-based detectors like LISA and Taiji, especially through joint observations.

Findings

A single LISA-like detector can measure soft displacement-memory signals at SNR ≥ 10.

Joint LISA-Taiji observations significantly improve measurement precision.

BBO could detect null memory from stellar-mass binary mergers.

Abstract

Gravitational-wave displacement memory is a remarkable and ubiquitous phenomenon predicted by general relativity, which has not yet been detected. Unlike the oscillatory components of gravitational waveforms, displacement memory is associated with soft gravitons, making it the only observable signal of its parent event at sufficiently low frequencies. Similarly, soft waveforms may arise from velocity and integrated-displacement memory. The simple and universal spectral shapes of soft waveforms also provide effective templates for matched filtering and parameter estimation. In this paper, we investigate the detection prospects for such soft memory signals with future space-based laser interferometers. As realistic examples, we examine the infrared spectral features of gravitational waves from moderately relativistic compact binary scattering and nearly equal-mass quasi-circular, non-precessing black hole mergers. In both cases, the low frequency spectrum can be described by a corrected soft waveform of displacement memory. The results of simulated Bayesian parameter estimation demonstrate that independent measurement of a soft displacement-memory signal with a single LISA-like detector is achievable at signal-to-noise ratios $\gtrsim 10$. The measurement precision can be significantly improved by joint observations with a LISA-Taiji network. A single BBO detector could be capable of separately measuring the null memory from stellar-mass compact binary mergers. We also evaluate the detectability of an idealized stochastic background of soft displacement-memory signals. Our results indicate that gravitational-wave bursts with memory can be promising targets for space-based interferometers.