Limits of vacuum-template subtraction for LISA massive black hole binary sources in realistic environments

TL;DR

This paper examines how gas accretion affects the subtraction of gravitational wave signals from massive black hole binaries in LISA data, revealing residuals that could bias other signal inferences.

Contribution

It provides a detailed analysis of the limitations of vacuum-template subtraction in realistic environments and offers a simple fitting function for the residuals based on population parameters.

Findings

Residual GW signals are unlikely to be distinguished from instrumental noise.

Imperfect subtraction can bias the inference of other signals.

Residuals depend on key population hyper-parameters.

Abstract

We investigate the impact of gravitational wave (GW) dephasing due to gas accretion on the subtraction of massive black hole (MBH) binary signals over 4 yr of LISA data in the context of the global-fit. Based on state of the art predictions for the population of merging MBHs, we show that imperfect subtraction with vacuum waveform templates leaves a GW residual with an SNR of $3.2^{+5.4}_{-1.9}\times \sqrt{f_{\rm Edd} \langle \dot n \rangle/(20\, {\rm yr}^{-1})}$, where $f_{\rm Edd}$ is the typical Eddington ratio and $\langle \dot n \rangle$ the mean merger rate of LISA MBH binaries. We characterize the dependence of the residual on key population hyper-parameters and provide a simple fitting function. {Finally, we consider the distinguishability of the residual as a stochastic signal by comparing with Bayesian power-law sensitivity curves, while discussing several additional detection and mitigation strategies. Overall, our analysis indicates that the residual is unlikely to be confidently distinguishable from instrumental noise while nevertheless being likely to bias the inference of other signals.