Search for nontensorial gravitational-wave backgrounds in the NANOGrav 15-year dataset

Abstract

The recent detection of a stochastic signal in the NANOGrav 15-year dataset has aroused great interest in uncovering its origin. However, the evidence for the Hellings-Downs correlations, a key signature of the gravitational-wave background (GWB) predicted by general relativity, remains inconclusive. In this paper, we search for an isotropic nontensorial GWB, allowed by general metric theories of gravity, in the NANOGrav 15-year dataset. Our analysis reveals a Bayes factor of approximately 2.5, comparing the quadrupolar (tensor transverse, TT) correlations to the scalar transverse (ST) correlations, suggesting that the ST correlations provide a comparable explanation for the observed stochastic signal in the NANOGrav data. We obtain the median and the 90% equal-tail amplitudes as ${\mathcal{A}}_{\mathrm{ST}}={7.8}_{\ensuremath{-}3.5}^{+5.1}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ at the frequency of $1/\mathrm{year}$. Furthermore, we find that the vector longitudinal (VL) and scalar longitudinal (SL) correlations are weakly and strongly disfavored by data, respectively, yielding upper limits on the amplitudes: ${\mathcal{A}}_{\mathrm{VL}}^{95%}\ensuremath{\lesssim}1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ and ${\mathcal{A}}_{\mathrm{SL}}^{95%}\ensuremath{\lesssim}7.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$. Lastly, we fit the NANOGrav data with the general transverse (GT) correlations parametrized by a free parameter $\ensuremath{\alpha}$. Our analysis yields $\ensuremath{\alpha}={1.74}_{\ensuremath{-}1.41}^{+1.18}$, thus excluding both the TT ($\ensuremath{\alpha}=3$) and ST ($\ensuremath{\alpha}=0$) models at the 90% confidence level.

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