Efficient computation of the gravitational wave spectrum emitted by eccentric massive black hole binaries in stellar environments

TL;DR

This paper introduces a fast, adaptable method to compute the gravitational wave spectrum from eccentric massive black hole binaries, accounting for stellar interactions and eccentricities, relevant for pulsar timing array observations.

Contribution

The authors develop an analytic, scalable approach to model the gravitational wave background from eccentric black hole binaries, incorporating stellar environment effects and spectrum features.

Findings

The method accurately models the GWB in PTA frequency range.

3-body stellar scattering minimally impacts the GWB in PTA band.

High eccentricities primarily cause low-frequency spectrum turnover.

Abstract

We present a fast and versatile method to calculate the characteristic spectrum $h_c$ of the gravitational wave background (GWB) emitted by a population of eccentric massive black hole binaries (MBHBs). We fit the spectrum of a reference MBHB with a simple analytic function and show that the spectrum of any other MBHB can be derived from this reference spectrum via simple scalings of mass, redshift and frequency. We then apply our calculation to a realistic population of MBHBs evolving via 3-body scattering of stars in galactic nuclei. We demonstrate that our analytic prescription satisfactorily describes the signal in the frequency band relevant to pulsar timing array (PTA) observations. Finally we model the high frequency steepening of the GWB to provide a complete description of the features characterizing the spectrum. For typical stellar distributions observed in massive galaxies, our calculation shows that 3-body scattering alone is unlikely to affect the GWB in the PTA band and a low frequency turnover in the spectrum is caused primarily by high eccentricities.