I led a collaborative project involving a search for an evidence of a gravitational wave background (GWB) with all the spatial correlations allowed by general metric theories of gravity using NANOGrav’s 12.5 year data set. The search found no substantial evidence in favor of the existence of such correlations.
The search found that the scalar-transverse (ST) correlations yield signal-to-noise ratios and Bayes factors that are higher than quadrupolar (tensor transverse, TT) correlations. Specifically, ST correlations yield a signal-to-noise ratio of 2.8 that are preferred over TT correlations (Hellings and Downs correlations) with Bayesian odds of about 20:1. However, the significance of ST correlations is reduced dramatically when one models the Solar System ephemeris systematics and/or remove pulsar J0030+0451 entirely from consideration. Even taking the nominal signal-to-noise ratios at face value, analyses of simulated data sets show that such values are not extremely unlikely to be observed in cases where only the usual TT modes are present in the GWB.
Funding Statement (additional comments about funding)
This work was partly supported by the George and Hannah Bolinger Memorial Fund in the College of Science at Oregon State University.
This work is supported in part by NASA under award number 80GSFC17M0002. We also acknowledge support received from NSF AAG award number 2009468. T.D. and M.T.L. are supported by an NSF Astronomy and Astrophysics grant (AAG) award No. 2009468. Portions of this work performed at NRL were supported by ONR 6.1 basic research funding.