An estimate of the stochastic gravitational wave background from the MassiveBlackII simulation

TL;DR

This paper estimates the stochastic gravitational wave background from super-massive black hole binaries using the MassiveBlackII simulation, considering different binary evolution models, and finds results consistent with current PTA limits.

Contribution

It provides new estimates of the SGWB characteristic strain using detailed simulation data and compares models with and without stellar and gas effects.

Findings

Characteristic strain at 1 yr^{-1} is approximately 6.9 x 10^{-16}.

Models including stellar scattering and gas effects show mild suppression of the SGWB.

Results are consistent with existing PTA upper limits.

Abstract

A population of super-massive black hole binaries is expected to generate a stochastic gravitational wave background (SGWB) in the pulsar timing array (PTA) frequency range of $10^{-9}$--$10^{-7}$ Hz. Detection of this signal is a current observational goal and so predictions of its characteristics are of significant interest. In this work we use super-massive black hole binary mergers from the MassiveBlackII simulation to estimate the characteristic strain of the stochastic background. We examine both a gravitational wave driven model of binary evolution and a model which also includes the effects of stellar scattering and a circumbinary gas disk. Results are consistent with PTA upper limits and similar to estimates in the literature. The characteristic strain at a reference frequency of $1 yr^{-1}$ is found to be $A_{yr^{-1}} = 6.9 \times 10^{-16}$ and $A_{yr^{-1}} = 6.4 \times 10^{-16}$ in the gravitational-wave driven and stellar scattering/gas disk cases, respectively. Using the latter approach, our models show that the SGWB is mildly suppressed compared to the purely gravitational wave driven model as frequency decreases inside the PTA frequency band.