PUB - Publikationen an der Universität Bielefeld

We present new limits on an isotropic stochastic gravitational-wavebackground (GWB) using a six pulsar dataset spanning 18 yr of observations fromthe 2015 European Pulsar Timing Array data release (Desvignes et al. in prep.).Performing a Bayesian analysis, we fit simultaneously for the intrinsic noiseparameters for each pulsar in this dataset, along with common correlatedsignals including clock, and Solar System ephemeris errors to obtain a robust95$\%$ upper limit on the dimensionless strain amplitude $A$ of the backgroundof $A<3.0\times 10^{-15}$ at a reference frequency of $1\mathrm{yr^{-1}}$ and aspectral index of $13/3$, corresponding to a background from inspirallingsuper-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 show thatperforming such an analysis when fixing the intrinsic noise parameters for theindividual pulsars leads to an erroneously stringent upper limit, by a factor$\sim 1.7$. We obtain a difference in the logarithm of the Bayesian evidencebetween models that include either a correlated background, or uncorrelatedcommon red noise of $-1.0 \pm 0.5$, indicating no support for the presence of acorrelated GWB in this dataset. We discuss the implications of our analysis forthe astrophysics of supermassive black hole binaries, and present 95$\%$ upperlimits on the string tension, $G\mu/c^2$, characterising a background producedby a cosmic string network for a set of possible scenarios, and for astochastic relic GWB. For a Nambu-Goto field theory cosmic string network, weset a limit $G\mu/c^2<1.3\times10^{-7}$, identical to that set by the PlanckCollaboration, combining Planck and high-$\ell$ Cosmic Microwave Backgrounddata from other experiments. (Abridged)