Gravitational wave background from binary systems

TL;DR

This paper reviews the mathematical characterization of the gravitational wave background from binary systems, introduces the overlap function for resolvability, and provides spectra predictions for various detectors, concluding that ground-based detectors will not observe a stochastic background.

Contribution

It generalizes the spectral energy density parameter to account for signal duration and observation time, and introduces the overlap function to improve background resolvability analysis.

Findings

The stochastic background from binaries is resolvable for ground-based detectors.

A semi-analytical formula for a(f) in stellar binaries is derived.

No stochastic background of binaries is expected for ground-based detectors.

Abstract

Basic aspects of the background of gravitational waves and its mathematical characterization are reviewed. The spectral energy density parameter $\Omega(f)$, commonly used as a quantifier of the background, is derived for an ensemble of many identical sources emitting at different times and locations. For such an ensemble, $\Omega(f)$ is generalized to account for the duration of the signals and of the observation, so that one can distinguish the resolvable and unresolvable parts of the background. The unresolvable part, often called confusion noise or stochastic background, is made by signals that cannot be either individually identified or subtracted out of the data. To account for the resolvability of the background, the overlap function is introduced. This function is a generalization of the duty cycle, which has been commonly used in the literature, in some cases leading to incorrect results. The spectra produced by binary systems (stellar binaries and massive black hole binaries) are presented over the frequencies of all existing and planned detectors. A semi-analytical formula for $\Omega(f)$ is derived in the case of stellar binaries (containing white dwarfs, neutron stars or stellar-mass black holes). Besides a realistic expectation of the level of background, upper and lower limits are given, to account for the uncertainties in some astrophysical parameters such as binary coalescence rates. One interesting result concerns all current and planned ground-based detectors (including the Einstein Telescope). In their frequency range, the background of binaries is resolvable and only sporadically present. In other words, there is no stochastic background of binaries for ground-based detectors.