How to Detect an Astrophysical Nanohertz Gravitational-Wave Background

TL;DR

This paper evaluates the effectiveness of standard statistical methods in detecting a nanohertz gravitational-wave background from supermassive black hole binaries using realistic simulated pulsar timing data, highlighting their robustness and limitations.

Contribution

It demonstrates that standard analysis techniques perform well on realistic simulations despite simplifying assumptions, and explores the impact of individual loud binaries on detection and spectral recovery.

Findings

Standard methods reliably detect the background in simulations.

Variance in detection significance due to different universe realizations.

Loud binaries can bias spectral estimates if not properly modeled.

Abstract

Analysis of pulsar timing data have provided evidence for a stochastic gravitational wave background in the nHz frequency band. The most plausible source of such a background is the superposition of signals from millions of supermassive black hole binaries. The standard statistical techniques used to search for such a background and assess its significance make several simplifying assumptions, namely: i) Gaussianity; ii) isotropy; and most often iii) a power-law spectrum. However, a stochastic background from a finite collection of binaries does not exactly satisfy any of these assumptions. To understand the effect of these assumptions, we test standard analysis techniques on a large collection of realistic simulated datasets. The dataset length, observing schedule, and noise levels were chosen to emulate the NANOGrav 15-year dataset. Simulated signals from millions of binaries drawn from models based on the Illustris cosmological hydrodynamical simulation were added to the data. We find that the standard statistical methods perform remarkably well on these simulated datasets, despite their fundamental assumptions not being strictly met. They are able to achieve a confident detection of the background. However, even for a fixed set of astrophysical parameters, different realizations of the universe result in a large variance in the significance and recovered parameters of the background. We also find that the presence of loud individual binaries can bias the spectral recovery of the background if we do not account for them.