Common-red-signal analysis with 24-yr high-precision timing of the European Pulsar Timing Array: inferences in the stochastic gravitational-wave background search

Abstract

We present results from the search for a stochastic gravitational-wave background (GWB) as predicted by the theory of General Relativity using six radio millisecond pulsars from the Data Release 2 (DR2) of the European Pulsar Timing Array (EPTA) covering a timespan up to 24 yr. A GWB manifests itself as a long-term low-frequency stochastic signal common to all pulsars, a common red signal (CRS), with the characteristic Hellings-Downs (HD) spatial correlation. Our analysis is performed with two independent pipelines, ENTERPRISE, and TEMPONEST+FORTYTWO, which produce consistent results. A search for a CRS with simultaneous estimation of its spatial correlations yields spectral properties compatible with theoretical GWB predictions, but does not result in the required measurement of the HD correlation, as required for GWB detection. Further Bayesian model comparison between different types of CRSs, including a GWB, finds the most favoured model to be the common uncorrelated red noise described by a power law with $A = 5.13_{-2.73}^{+4.20} \times 10^{-15}$ and $\gamma = 3.78_{-0.59}^{+0.69}$ (95 per cent credible regions). Fixing the spectral index to γ = 13/3 as expected from the GWB by circular, inspiralling supermassive black hole binaries results in an amplitude of $A =2.95_{-0.72}^{+0.89} \times 10^{-15}$. We implement three different models, BAYESEPHEM, LINIMOSS, and EPHEMGP, to address possible Solar system ephemeris (SSE) systematics and conclude that our results may only marginally depend on these effects. This work builds on the methods and models from the studies on the EPTA DR1. We show that under the same analysis framework the results remain consistent after the data set extension.

Noise analysis in the European Pulsar Timing Array data release 2 and its implications on the gravitational-wave background search

“…When the Bayes factor is not informative (close to one) we make the selection based either on the simplicity of the model or on the basis of computational efficiency with very few exceptions, which we explicitly discuss below. Once we had customized the noise model for each pulsar, we reproduced the main results of Chen et al ( 2021 ), namely we confirmed the high statistical significance of a common red signal, but without sufficient evidence for a GWB induced Hellings-Downs correlation. We want to emphasize that the final noise model used for each pulsar was quite different from the simpler model assumed in Chen et al ( 2021 ) for all pulsars.…”

“…Once we had customized the noise model for each pulsar, we reproduced the main results of Chen et al ( 2021 ), namely we confirmed the high statistical significance of a common red signal, but without sufficient evidence for a GWB induced Hellings-Downs correlation. We want to emphasize that the final noise model used for each pulsar was quite different from the simpler model assumed in Chen et al ( 2021 ) for all pulsars. The noise model selection method described in this paper will be used on the extended (25 pulsars) EPTA data.…”

“…The EPTA Data Release 2 (DR2) -six pulsars data set (Chen et al 2021 ) -comprises up to 24 yr of high cadence observations of PSRs J0613 −0200, J1012 + 5307, J1600 −3053, J1713 + 0747, J1744 −1134, and J1909 −3744. These pulsars are observed in singledish mode at four European radio telescopes: the Effelsberg 100-m radio telescope (EFF), the Nan c ¸ay Radio telescope (NRT), the Lo v ell Telescope at the Jodrell Bank Observatory (JBO), and the Westerbork Synthesis Radio Telescope (WSRT).…”

“…The three historical PTAs: Parkes pulsar timing array (PPTA; Kerr et al 2020 ), the North American Nanohertz Observatory for Gra vitational-wa ves (NANOGra v; Alam et al 2021 ), and the European pulsar timing array (EPTA; Desvignes et al 2016 ) cooperate with the emerging Indian pulsar timing array (InPTA) 1 as the International pulsar timing array (IPTA; Verbiest et al 2016 ;Perera et al 2019 ). The PPTA, NANOGrav, and the EPTA have recently published results produced using with their own independently processed data sets that show consistent evidence for the presence of a red (time-correlated) signal common among pulsars (Arzoumanian et al 2020 ;Chen et al 2021 ;Goncharov et al 2021a ) but without the necessary evidence for Hellings-Downs correlations, which would confirm the detection of a GWB. Encouraged by these promising results, the IPTA is planning to combine extended data sets from each PTA collaboration, in the hope that the GW nature of the observed signal can be confirmed (or disproved).…”

“…This is the main moti v ation behind this paper. We consider this paper as a companion of Chen et al ( 2021 ), where the main focus was on finding and characterizing the common red signal (CRS) using two independent pipelines.…”