Muffled Murmurs: Environmental effects in the LISA stochastic signal from stellar-mass black hole binaries

TL;DR

This paper models how environmental factors like gas dynamical friction and accretion influence the stochastic gravitational-wave background from stellar-mass black hole binaries detectable by LISA, and assesses their observability.

Contribution

It introduces a phenomenological model for environmental effects on the SGWB and evaluates LISA's ability to detect or constrain these effects using Bayesian inference.

Findings

LISA cannot detect accretion effects even at high Eddington ratios.

LISA can constrain gas density to rac{rac{7.6 imes 10^{-10}}{ ext{g} \, ext{cm}^{-3}}}

Dynamical friction effects are detectable at densities rac{rac{10^{-10}-10^{-9}}{ ext{g} \, ext{cm}^{-3}}} with high Bayes factors.

Abstract

The population of unresolved stellar-mass black hole binaries (sBBHs) is expected to produce a stochastic gravitational-wave background (SGWB) potentially detectable by the Laser Interferometer Space Antenna (LISA). In this work, we compute the imprint of astrophysical environmental effect--such as gas dynamical friction and accretion--on this background. Using the sBBHs population constraints obtained by the LIGO--Virgo--Kagra collaboration, we compute the expected SGWB and develop a phenomenological parametric model that can accurately capture the effect of dynamical friction and accretion. Using our model, we perform Bayesian inference on simulated signals to assess the detectability of these environmental effects. We find that even for large injected values of the Eddington ratio, the effect of accretion in the SGWB is undetectable by LISA. However, LISA will be able to constrain the effect of dynamical friction with an upper bound on the gas density of $\rho \lesssim 7.6 \times 10^{-10} \mathrm{g \, cm^{-3}}$, thus probing the sBBH environment forming in typical thin accretion disks around Active Galactic Nuclei (AGNs). For injected densities of $\rho \sim 10^{-10}-10^{-9} \mathrm{g} \, \mathrm{cm}^{-3}$, dynamical friction effects can be well measured and clearly distinguished from vacuum, with Bayes factors reaching up to $\sim$ 60, even when the Galactic foreground is included.