Publication Date:
2015-09-02

Description:
We present new limits on an isotropic stochastic gravitational-wave background (GWB) using a six pulsar data set 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 per cent upper limit on the dimensionless strain amplitude A of the background of A 〈 3.0 x 10 –15 at a reference frequency of 1 yr –1 and a spectral index of 13/3, corresponding to a background from inspiralling supermassive black hole binaries, constraining the GW energy density to gw ( f ) h 2 〈 1.1 x 10 –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 ~5 x 10 –9 Hz. Finally, we discuss the implications of our analysis for the astrophysics of supermassive black hole binaries, and present 95 per cent upper limits on the string tension, G μ/ c 2 , characterizing 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 μ/ c 2 〈 1.3 x 10 –7 , identical to that set by the Planck Collaboration, when combining Planck and high- cosmic microwave background data from other experiments. For a stochastic relic background, we set a limit of $\Omega ^\mathrm{relic}_\mathrm{gw}(f)h^2 〈 1.2 \times 10^{-9}$ , a factor of 9 improvement over the most stringent limits previously set by a pulsar timing array.

Print ISSN:
0035-8711

Electronic ISSN:
1365-2966

Topics:
Physics

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