Non-tensorial Gravitational Wave Background in NANOGrav 12.5-Year Data Set

TL;DR

This study searches for non-tensorial gravitational wave backgrounds in NANOGrav data, finding strong evidence for scalar transverse correlations but no significant evidence for other polarization modes, expanding tests of gravity.

Contribution

First search for an isotropic non-tensorial gravitational-wave background in pulsar timing data, exploring general metric theories of gravity and identifying scalar transverse mode signals.

Findings

Strong Bayesian evidence for scalar transverse correlations

No significant evidence for tensor, vector, or scalar longitudinal modes

Estimated amplitude of scalar transverse mode at 1/year frequency

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 $107\pm 7$, 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 $\mathcal{A}_{\mathrm{ST}}= 1.06^{+0.35}_{-0.28} \times 10^{-15}$, or equivalently the energy density parameter per logarithm frequency is $\Omega_{\mathrm{GW}}^{\mathrm{ST}} = 1.54^{+1.21}_{-0.71} \times 10^{-9}$, at frequency of 1/year.