The implications of stochastic gas torques for asymmetric binaries in the LISA band

TL;DR

This study uses hydrodynamic simulations to assess how stochastic gas torques affect gravitational waveforms from asymmetric black-hole binaries in the LISA band, highlighting potential biases in disk property estimates.

Contribution

It introduces a method to evaluate the impact of stochastic torque variability on gravitational wave analysis, improving the realism of models for LISA observations.

Findings

Bayesian analysis shows no bias in binary parameters despite stochastic torques.

Estimates of accretion disk properties can be biased by stochastic variability.

Simplified torque models are effective for binary parameter estimation, but caution is needed for disk properties.

Abstract

Gravitational waves from asymmetric mass-ratio black-hole binaries carry unique information about their astrophysical environment. For instance, the Laser Interferometer Space Antenna (LISA) could potentially measure the amplitude and slope of gas torques in binaries embedded in the accretion disks of Active Galactic Nuclei, helping differentiate competing accretion disk models. However, this relies on simplified analytic models, which do not account for the stochastic variability of torques seen in hydrodynamic simulations. In this work, we use hydrodynamic simulations to create gravitational waveforms for extreme and intermediate mass-ratio inspirals in the LISA band. We then analyze these simulated waveforms using simpler templates that assume analytic torques, without stochastic time variability. By performing realistic Bayesian parameter estimation, we find no bias at 90% confidence in the binary parameters; however, estimates of accretion disk parameters, such as torque amplitude and slope, may be biased. Typically, the posterior distribution is centered around the average value of the torques, but when stochastic variability is large, the posterior can indicate no torques, even though they are present in the simulation. Our results suggest that while simplified analytic torque models work well for estimating binary parameters, caution is needed when using them to infer properties of the accretion disk. This work moves towards a more realistic assessment of one of the LISA science objectives, i.e., probing the properties of the astrophysical environments of black holes.