Eccentricity effects on the supermassive black hole gravitational wave background

TL;DR

This paper investigates how eccentricity influences the gravitational wave background from supermassive black hole binaries, introducing a semi-analytic method to model the spectrum and analyzing implications for pulsar timing array observations.

Contribution

The authors developed a fast semi-analytic approach to compute SMBH binary GW spectra, including spectral fluctuations and correlations, accounting for eccentricity effects and environmental influences.

Findings

Highly eccentric binaries are preferred when GW emission dominates orbital decay.

Environmental effects can reduce initial eccentricity, but circular binaries remain mildly disfavored.

The method improves modeling accuracy for GW signals from SMBH binaries.

Abstract

We studied how eccentricity affects the gravitational wave (GW) spectrum from supermassive black hole (SMBH) binaries. We developed a fast and accurate semi-analytic method for computing the GW spectra, the distribution for the spectral fluctuations and the correlations between different frequencies. As GW emission circularizes binaries, the suppression of the signal strength due to eccentricity is relevant for signals from wider binaries emitting at lower frequencies. Such a feature is present in the signal observed at pulsar timing arrays. We found that when orbital decay of the SMBH binaries is driven by GWs only, the shape of the observed signal preferred highly eccentric binaries $\langle e \rangle_{2\,{\rm nHz}} = 0.83^{+0.04}_{-0.05}$. However, when environmental effects were included, the initial eccentricity could be significantly lowered, yet the scenario with purely circular binaries was still mildly disfavored.