Gravitational wave background from extreme mass ratio inspirals

TL;DR

This paper investigates the gravitational wave background generated by unresolved EMRIs, showing it could significantly impact LISA's sensitivity and emphasizing the need for better astrophysical understanding of EMRI formation.

Contribution

It provides a comprehensive analysis of the stochastic gravitational wave background from EMRIs across various formation scenarios, highlighting its detectability and potential to affect LISA's performance.

Findings

The EMRI background is detectable by LISA with high signal-to-noise ratios.

In some models, the background can dominate or erase LISA's sensitivity in certain frequency ranges.

Understanding EMRI astrophysics is crucial for accurate LISA data interpretation.

Abstract

Extreme mass ratio inspirals (EMRIs), i.e. binary systems comprised by a compact stellar-mass object orbiting a massive black hole, are expected to be among the primary gravitational wave (GW) sources for the forthcoming LISA mission. The astrophysical processes leading to the formation of such systems still remain poorly understood, resulting into large uncertainties in the predicted cosmic rate of these sources, spanning at least three orders of magnitude. As LISA can individually resolve mostly EMRIs up to $z\gtrsim1$, the ensemble of signals below its detection threshold will add up incoherently forming an unresolved confusion noise, which can be formally described as a stochastic background. We perform an extensive study of this background by considering a collection of astrophysically motivated EMRI formation scenarios, spanning current uncertainties. We find that, for most astrophysical models, this signal is easily detectable by LISA, with signal to noise ratios of several hundreds. In fiducial EMRI models -- predicting hundreds of EMRI detections during mission operations -- the background level is comparable to the LISA noise, affecting the performance of the instrument around 3 mHz. In extreme cases, this background can even "erase" the whole LISA sensitivity bucket in the 2-10 mHz frequency range. This points to the need of a better understanding of EMRIs' astrophysics for a full assessment of the LISA mission potential.