Non-tensorial gravitational wave background in NANOGrav 12.5-year data set

Abstract

We perform the first search for an isotropic non-tensorial gravitational-wave background (GWB) allowed in general metric theories of gravity in the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 12.5-year data set. By modeling the GWB as a power-law spectrum, we find strong Bayesian indication for a spatially correlated process with scalar transverse (ST) correlations whose Bayes factor versus the spatially uncorrelated common-spectrum process is 107 ± 7, but no statistically significant evidence for the tensor transverse, vector longitudinal, and scalar longitudinal polarization modes. The median and the 90% equal-tail amplitudes of ST mode are $$\cal{A}_{\text{ST}}=1.06_{-0.28}^{+0.35}\times10^{-15}$$ , or equivalently the energy density parameter per logarithm frequency is $$\Omega_{\text{GW}}^{\text{ST}}=1.54_{-0.71}^{+1.21}\times10^{-9}$$ , at frequency of 1/year.