DOES A "STOCHASTIC" BACKGROUND OF GRAVITATIONAL WAVES EXIST IN THE PULSAR TIMING BAND?

We investigate the effects of gravitational waves (GWs) from a simulatedpopulation of binary supermassive black holes (SMBHs) on pulsar timingarray data sets. We construct a distribution describing the binary SMBHpopulation from an existing semi-analytic galaxy formation model. Usingrealizations of the binary SMBH population generated from thisdistribution, we simulate pulsar timing data sets with GW-inducedvariations. We find that the statistics of these variations do notcorrespond to an isotropic, stochastic GW background. The ''Hellings{\amp} Downs'' correlations between simulated data sets for differentpulsars are recovered on average, though the scatter of the correlationestimates is greater than expected for an isotropic, stochastic GWbackground. These results are attributable to the fact that just a fewGW sources dominate the GW-induced variations in every Fourier frequencybin of a five-year data set. Current constraints on the amplitude of theGW signal from binary SMBHs will be biased. Individual binary systemsare likely to be detectable in five-year pulsar timing array data setswhere the noise is dominated by GW-induced variations. Searches for GWsin pulsar timing array data therefore need to account for the effects ofindividual sources of GWs.