Does a 'stochastic' background of gravitational waves exist in the pulsar timing band?

TL;DR

This study simulates gravitational wave effects from binary super-massive black holes on pulsar timing arrays, revealing that the background is dominated by a few sources rather than being isotropic and stochastic, affecting detection strategies.

Contribution

It introduces a realistic simulation of GW signals from SMBH binaries, showing the dominance of individual sources over a stochastic background in pulsar timing data.

Findings

GW variations are dominated by few sources, not a stochastic background.

Hellings & Downs correlations are recovered but with greater scatter.

Current GW amplitude constraints may be biased due to source dominance.

Abstract

We investigate the effects of gravitational waves (GWs) from a simulated population of binary super-massive black holes (SMBHs) on pulsar timing array datasets. We construct a distribution describing the binary SMBH population from an existing semi-analytic galaxy formation model. Using realizations of the binary SMBH population generated from this distribution, we simulate pulsar timing datasets with GW-induced variations. We find that the statistics of these variations do not correspond to an isotropic, stochastic GW background. The "Hellings & Downs" correlations between simulated datasets for different pulsars are recovered on average, though the scatter of the correlation estimates is greater than expected for an isotropic, stochastic GW background. These results are attributable to the fact that just a few GW sources dominate the GW-induced variations in every Fourier frequency bin of a 5-year dataset. Current constraints on the amplitude of the GW signal from binary SMBHs will be biased. Individual binary systems are likely to be detectable in 5-year pulsar timing array datasets where the noise is dominated by GW-induced variations. Searches for GWs in pulsar timing array data therefore need to account for the effects of individual sources of GWs.