A Stochastic Gravitational Wave Background in LISA from Unresolved White Dwarf Binaries in the Large Magellanic Cloud

TL;DR

This paper demonstrates that unresolved white dwarf binaries in the Large Magellanic Cloud will create a significant stochastic gravitational wave background detectable by LISA, using Bayesian inference and simulated data analysis.

Contribution

It introduces a novel analysis of the LMC's unresolved white dwarf binaries as a detectable stochastic gravitational wave background using Bayesian methods.

Findings

Unresolved DWDs in the LMC produce a significant SGWB for LISA.

Bayesian spherical harmonic analysis can detect anisotropic SGWB signals.

Spectral separation from the Galactic foreground is feasible.

Abstract

The Laser Interferometer Space Antenna (LISA) is expected to detect a wide variety of gravitational wave sources in the mHz band. Some of these signals will elude individual detection, instead contributing as confusion noise to one of several stochastic gravitational-wave backgrounds (SGWBs) -- notably including the `Galactic foreground', a loud signal resulting from the superposition of millions of unresolved double white dwarf binaries (DWDs) in the Milky Way. It is possible that similar, weaker SGWBs will be detectable from other DWD populations in the local universe, including the Large Magellanic Cloud (LMC). We use the Bayesian LISA Inference Package ($\tt{BLIP}$) to investigate the possibility of an anisotropic SGWB generated by unresolved DWDs in the LMC. To do so, we compute the LMC SGWB from a realistic DWD population generated via binary population synthesis, simulate four years of time-domain data with $\tt{BLIP}$ comprised of stochastic contributions from the LMC SGWB and the LISA detector noise, and analyze this data with $\tt{BLIP}$'s spherical harmonic anisotropic SGWB search. We also consider the case of spectral separation from the Galactic foreground. We present the results of these analyses and show, for the first time, that the unresolved DWDs in the LMC will comprise a significant SGWB for LISA.