Direct detection of low-frequency gravitational waves (10−9−10−8 Hz) is the main goal of pulsar timing array (PTA) projects. One of the main targets for the PTAs is to measure the stochastic background of gravitational waves (GWB) whose characteristic strain is expected to approximately follow a power-law of the form hc(f)=A(f/yr−1)α, where f is the gravitational-wave frequency. In this paper we use the current data from the European PTA to determine an upper limit on the GWB amplitude A as a function of the unknown spectral slope α with a Bayesian algorithm, by modelling the GWB as a random Gaussian process. For the case α=−2/3, which is expected if the GWB is produced by supermassive black-hole binaries, we obtain a 95% confidence upper limit on A of 6×10−15, which is 1.8 times lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our approach to the data analysis incorporates the multi-telescope nature of the European PTA and thus can serve as a useful template for future intercontinental PTA collaborations.
@article{arxiv.1103.0576,
title = {Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data},
author = {R. van Haasteren and Y. Levin and G. H. Janssen and K. Lazaridis and M. Kramer B. W. Stappers and G. Desvignes and M. B. Purver and A. G. Lyne and R. D. Ferdman and A. Jessner and I. Cognard and G. Theureau and N. D'Amico and A. Possenti and M. Burgay and A. Corongiu and J. W. T. Hessels and R. Smits and J. P. W. Verbiest},
journal= {arXiv preprint arXiv:1103.0576},
year = {2015}
}