We have searched for continuous gravitational wave (CGW) signals produced by individually resolvable, circular supermassive black hole binaries (SMBHBs) in the latest EPTA dataset, which consists of ...ultra-precise timing data on 41 millisecond pulsars. We develop frequentist and Bayesian detection algorithms to search both for monochromatic and frequency-evolving systems. None of the adopted algorithms show evidence for the presence of such a CGW signal, indicating that the data are best described by pulsar and radiometer noise only. Depending on the adopted detection algorithm, the 95\% upper limit on the sky-averaged strain amplitude lies in the range \(6\times 10^{-15}<A<1.5\times10^{-14}\) at \(5{\rm nHz}<f<7{\rm nHz}\). This limit varies by a factor of five, depending on the assumed source position, and the most constraining limit is achieved towards the positions of the most sensitive pulsars in the timing array. The most robust upper limit -- obtained via a full Bayesian analysis searching simultaneously over the signal and pulsar noise on the subset of ours six best pulsars -- is \(A\approx10^{-14}\). These limits, the most stringent to date at \(f<10{\rm nHz}\), exclude the presence of sub-centiparsec binaries with chirp mass \(\cal{M}_c>10^9\)M\(_\odot\) out to a distance of about 25Mpc, and with \(\cal{M}_c>10^{10}\)M\(_\odot\) out to a distance of about 1Gpc (\(z\approx0.2\)). We show that state-of-the-art SMBHB population models predict \(<1\%\) probability of detecting a CGW with the current EPTA dataset, consistent with the reported non-detection. We stress, however, that PTA limits on individual CGW have improved by almost an order of magnitude in the last five years. The continuing advances in pulsar timing data acquisition and analysis techniques will allow for strong astrophysical constraints on the population of nearby SMBHBs in the coming years.
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.
