Abstract
Abstract We search NANOGrav’s 12.5 yr data set for evidence of a gravitational-wave background (GWB) with all the spatial correlations allowed by general metric theories of gravity. We find no substantial evidence in favor of the existence of such correlations in our data. We find that scalar-transverse (ST) correlations yield signal-to-noise ratios and Bayes factors that are higher than quadrupolar (tensor-transverse, TT) correlations. Specifically, we find ST correlations with 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 we include modeling of 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. In the absence of a detection of any polarization mode of gravity, we place upper limits on their amplitudes for a spectral index of γ = 5 and a reference frequency of f yr = 1 yr−1. Among the upper limits for eight general families of metric theories of gravity, we find the values of A TT 95 % = ( 9.7 ± 0.4 ) × 10 − 16 and A ST 95 % = ( 1.4 ± 0.03 ) × 10 − 15 for the family of metric spacetime theories that contain both TT and ST modes.
Highlights
Pulsar timing experiments (Sazhin 1978; Detweiler 1979) allow us to explore the low-frequency (∼ 1100 nHz) part of the gravitational-wave (GW) spectrum
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 transverse mode of gravity (TT mode) are present in the gravitational wave background (GWB)
We only perform upper limit analyses for the vector and scalar-longitudinal modes of gravity. This is for three reasons: i) large correlations at small angular separations predicted for the longitudinal (VL, scalar longitudinal (SL)) polarization modes, are absent in the current data set, ii) as shown in Fig. 3, the values of the overlap reduction function (ORF) for the SL mode are very sensitive to pulsar distances for most angular separations, and, iii) the addition of frequency-dependent terms to our current detection pipeline required for the SL mode demands significant modifications, testing, and simulations which are outside the scope of this work
Summary
Pulsar timing experiments (Sazhin 1978; Detweiler 1979) allow us to explore the low-frequency (∼ 1100 nHz) part of the gravitational-wave (GW) spectrum. We start our analyses by studying simulated PTA datasets similar to NANOGrav’s 12.5-year dataset (Pol et al 2021) and show that for current datasets (with tens of pulsars having observational baselines less than 15 years and for typical amplitudes of the GWB signal ∼ 2 × 10−15), the correlations induced by transverse modes of GWs can be hard to distinguish from one another. These results are shown first to set our expectations for our analyses of the dataset in hand as well as future datasets.
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