The International Pulsar Timing Array second data release: search for an isotropic gravitational wave background

We report on the search for an isotropic stochastic gravitational wave background in the second data release of the International Pulsar Timing Array (IPTA), a global collaboration synthesizing decadal-length pulsar-timing campaigns in North America, Europe, and Australia.  We find strong evidence for a spectrally-similar, low-frequency, stochastic process with amplitude A = 3.8^+6.3_-2.5 × 10^-15 at f = 1/yr and spectral index α = -0.5 ± 0.5, where uncertainies represent 95% credible intervals.  When assuming a spectral index of α = -2/3, as expected from a population of inspiralling supermassive black hole binaries, the recovered amplitude is A = 2.8^+1.2_-0.8 × 10^-15.  No significant evidence was found for the Hellings-Downs correlations which would indicate a gravitational wave origin.  This combined data set, which was built from older data, produces results in agreement with more recent individual data sets from the NANOGrav, PPTA, and EPTA collaborations.  This shows that future IPTA data releases can deliver increased sensitivity to gravitational waves relative to their constituent parts.