Searching for isotropic stochastic gravitational-wave background in the international pulsar timing array second data release

Abstract We search for isotropic stochastic gravitational-wave background (SGWB) in the International Pulsar Timing Array second data release. By modeling the SGWB as a power-law, we find very strong Bayesian evidence for a common-spectrum process, and further this process has scalar transverse (ST) correlations allowed in general metric theory of gravity as the Bayes factor in favor of the ST-correlated process versus the spatially uncorrelated common-spectrum process is 30 ± 2. The median and the 90% equal-tail amplitudes of ST mode are  ST = 1.29 − 0.44 + 0.51 × 10 − 15 , or equivalently the energy density parameter per logarithm frequency is Ω GW ST = 2.31 − 1.30 + 2.19 × 10 − 9 , at frequency of 1 year −1 . However, we do not find any statistically significant evidence for the tensor transverse (TT) mode and then place the 95% upper limits as  TT 3.95 × 10 − 15 , or equivalently Ω GW TT 2.16 × 10 − 9 , at frequency of 1 year −1 .