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 $30\pm 2$. The median and the $90\%$ equal-tail amplitudes of ST mode are ${\mathcal{A}}_{\mathrm{ST}}=1.29_{-0.44}^{+0.51}\times 10^{-15}$, or equivalently the energy density parameter per logarithm frequency is ${\Omega }_{\mathrm{GW}}^{\mathrm{ST}}=2.31_{-1.30}^{+2.19}\times 10^{-9}$, at frequency of 1/year. However, we do not find any statistically significant evidence for the tensor transverse (TT) mode and then place the $95\%$ upper limits as ${\mathcal{A}}_{\mathrm{TT}}<3.95\times 10^{-15}$, or equivalently ${\Omega }_{\mathrm{GW}}^{\mathrm{TT}}<2.16\times 10^{-9}$, at frequency of 1/year.

Comments: 7 pages, 4 figures, 3 tables