The LISA Gravitational Wave Foreground: A Study of Double White Dwarfs

TL;DR

This study analyzes the impact of double white dwarf binaries, including mass transferring systems, on the LISA gravitational wave foreground, revealing their significant contribution at higher frequencies and identifying a large resolvable population.

Contribution

It provides the first detailed assessment of Roche-Lobe Overflow binaries' influence on the LISA foreground, emphasizing the importance of including mass transferring binaries in models.

Findings

RLOF binaries significantly affect the foreground noise above 6 mHz.

Approximately 11,300 binaries will be resolvable by LISA with SNR > 5.

Mass transferring binaries are crucial for accurate foreground noise estimation.

Abstract

Double white dwarfs are expected to be a source of confusion-limited noise for the future gravitational wave observatory LISA. In a specific frequency range, this 'foreground noise' is predicted to rise above the instrumental noise and hinder the detection of other types of signals, e.g., gravitational waves arising from stellar mass objects inspiraling into massive black holes. In many previous studies only detached populations of compact object binaries have been considered in estimating the LISA gravitational wave foreground signal. Here, we investigate the influence of compact object detached and Roche-Lobe Overflow Galactic binaries on the shape and strength of the LISA signal. Since >99% of remnant binaries which have orbital periods within the LISA sensitivity range are white dwarf binaries, we consider only these binaries when calculating the LISA signal. We find that the contribution of RLOF binaries to the foreground noise is negligible at low frequencies, but becomes significant at higher frequencies, pushing the frequency at which the foreground noise drops below the instrumental noise to >6 mHz. We find that it is important to consider the population of mass transferring binaries in order to obtain an accurate assessment of the foreground noise on the LISA data stream. However, we estimate that there still exists a sizeable number (~11300) of Galactic double white dwarf binaries which will have a signal-to-noise ratio >5, and thus will be potentially resolvable with LISA. We present the LISA gravitational wave signal from the Galactic population of white dwarf binaries, show the most important formation channels contributing to the LISA disc and bulge populations and discuss the implications of these new findings.