Gravitational wave energy spectral density properties from BPASS Galactic binary population in the Milky Way galaxy

TL;DR

The paper addresses the characterization of the Galactic gravitational-wave foreground in the LISA band by synthesizing BPASS-based binary populations within a Milky Way–like FIRE-2 galaxy. It compares three spectral shapes—single power-law, broken power-law, and single-peak—for the GW energy density and uses Bayesian inference to estimate the corresponding parameters across six Galactic populations and their total, revealing substantial variability in the ESD shapes and dominance patterns. The analysis finds that white-dwarf binaries and neutron star–white dwarf systems primarily shape the LISA-band foreground, while BHNS signals are largely absent above 2 mHz, and that a simple single power-law often does not capture the full spectral structure. These results highlight the need for flexible spectral modeling and more realistic noise characterizations to accurately separate the Galactic foreground from potential cosmological signals in LISA data.

Abstract

We analyse the energy spectral density properties of Gravitational waves from Galactic binary populations in the~\text{mHz} band targeted by the Laser Interferometer Space Antenna mission. Our analysis is based on combining BPASS with a Milky Way analogue galaxy from the Feedback In Realistic Environment (FIRE) simulations and the GWs these populations emit. Our investigation compares different functional forms of gravitational wave (GW) ESDs, namely the single power-law, broken power-law, and single-peak models, revealing disparities within and among Galactic binary populations. We estimate the ESDs for six different Galactic binary populations and the ESD of the total Galactic binary population for LISA. Employing a single power-law model, we predict a total Galactic binary GW signal amplitude $α$ = $2.0^{+0.2}_{-0.2} \times 10^{-8}$ and a slope $β$ = $-2.64 ^{+0.03}_{-0.04}$ and the ESD $\rm h^2 Ω_{GW}$ = $1.1 ^{+0.1}_{-0.1} \times 10^{-9}$ at 3~\text{mHz}. For the Galactic WDB binary GW signal $α= 1^{+0.02}_{-0.02} \times 10^{-10}$, $β= -1.56 ^{+0.03}_{-0.03}$ and $\rm h^2 Ω_{GW} = 18 ^{+1}_{-1} \times 10^{-12}$. Our analysis underscores the importance of accurate noise parameter estimation and highlights the complexities of modelling realistic observations, prompting future exploration into more flexible models.

Figures (22)

• Figure 1: Modulated signals over one-year LISA mission for a single channal A, signal frequencies ranging from 0.1 mHz to 0.1 Hz.
• Figure 2: Response function $\overline{F}_{A}(t)$ averaged signals over one-year LISA mission, signal frequencies ranging from 0.1 mHz to 0.1 Hz.
• Figure 3: The periodogram of the GW signal of different Galactic binary populations.
• Figure 4: Four-parameter Bayesian parametric single PL models for estimating ESD over the full LISA frequency range of 0.1 mHz to 30 mHz. The grey lines represent the periodogram of the GW signal. The orange lines represent the median ESD estimates, and the HDI is taken as 90 per cent is shaded in blue.
• Figure 5: Estimated ESD from mean parameter values for all Galactic populations, black line is the LISA sensitivity evaluated as $h^2\Omega_n(f)$. Top: assuming the GW signal ESD is in the form of a single PL. Middle: broken PL model. Bottom: single-peak model.