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

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 A(ST) = 1.06(-0.28)(+0.35) x 10(-15), or equivalently the energy density parameter per logarithm frequency is Omega(ST)(GW) = 1.54(-0.71)(+1.21) x 10(-9), at frequency of 1/year.

Automated summary

This study in the NANOGrav data found strong Bayesian evidence for an isotropic non-tensorial gravitational-wave background with scalar transverse correlations. However, no statistically significant evidence was found for other polarization modes.