Consistency of the Parkes Pulsar Timing Array Signal with a Nanohertz Gravitational-wave Background

Pulsar timing array experiments have recently reported strong evidence for a common-spectrum stochastic process with a strain spectral index consistent with that expected of a nanohertz-frequency gravitational-wave background, but with negligible yet non-zero evidence for spatial correlations required for a definitive detection. However, it was pointed out by the Parkes Pulsar Timing Array (PPTA) collaboration that the same models used in recent analyses resulted in strong evidence for a common-spectrum process in simulations where none is present. In this work, we introduce a methodology to distinguish pulsar power spectra with the same amplitude from noise power spectra of similar but distinct amplitudes. The former is the signature of a spatially uncorrelated pulsar term of a nanohertz gravitational-wave background, whereas the latter could represent ensemble pulsar noise properties. We test the methodology on simulated data sets. We find that the reported common process in PPTA pulsars is indeed consistent with the spectral feature of a pulsar term. We recommend this methodology as one of the validity tests that the real astrophysical and cosmological backgrounds should pass, as well as for inferences about the spatially uncorrelated component of the background.

Automated summary

Pulsar timing array experiments provide strong evidence for a common-spectrum stochastic process consistent with a nanohertz-frequency gravitational-wave background, but further research is needed to detect spatial correlations. A methodology to distinguish pulsar power spectra from noise power spectra is introduced and tested on simulated data sets, finding that the reported common process in pulsars is consistent with the spectral feature of a pulsar term. This methodology is recommended as a validity test for astrophysical and cosmological backgrounds.