European Pulsar Timing Array Limits On An Isotropic Stochastic Gravitational-Wave Background

TL;DR

This paper reports new upper limits on an isotropic stochastic gravitational-wave background using 18 years of pulsar timing data, constraining astrophysical sources like supermassive black hole binaries and cosmic strings.

Contribution

The study provides the most stringent pulsar timing array limits to date on gravitational-wave backgrounds from supermassive black hole binaries and cosmic strings, using a comprehensive Bayesian analysis.

Findings

Upper limit on gravitational-wave strain amplitude: A<3.0×10^{-15} at 1 yr^{-1}

Constraint on cosmic string tension: Gμ/c^2<1.3×10^{-7}

Improved limits on relic gravitational-wave background: Ω_gw<1.2×10^{-9}

Abstract

We present new limits on an isotropic stochastic gravitational-wave background (GWB) using a six pulsar dataset spanning 18 yr of observations from the 2015 European Pulsar Timing Array data release. Performing a Bayesian analysis, we fit simultaneously for the intrinsic noise parameters for each pulsar, along with common correlated signals including clock, and Solar System ephemeris errors, obtaining a robust 95$\%$ upper limit on the dimensionless strain amplitude $A$ of the background of $A<3.0\times 10^{-15}$ at a reference frequency of $1\mathrm{yr^{-1}}$ and a spectral index of $13/3$, corresponding to a background from inspiralling super-massive black hole binaries, constraining the GW energy density to $\Omega_\mathrm{gw}(f)h^2 < 1.1\times10^{-9}$ at 2.8 nHz. We also present limits on the correlated power spectrum at a series of discrete frequencies, and show that our sensitivity to a fiducial isotropic GWB is highest at a frequency of $\sim 5\times10^{-9}$~Hz. Finally we discuss the implications of our analysis for the astrophysics of supermassive black hole binaries, and present 95$\%$ upper limits on the string tension, $G\mu/c^2$, characterising a background produced by a cosmic string network for a set of possible scenarios, and for a stochastic relic GWB. For a Nambu-Goto field theory cosmic string network, we set a limit $G\mu/c^2<1.3\times10^{-7}$, identical to that set by the {\it Planck} Collaboration, when combining {\it Planck} and high-$\ell$ Cosmic Microwave Background data from other experiments. For a stochastic relic background we set a limit of $\Omega^\mathrm{relic}_\mathrm{gw}(f)h^2<1.2 \times10^{-9}$, a factor of 9 improvement over the most stringent limits previously set by a pulsar timing array.