A clearer view of gravitational-wave signals in pulsar timing arrays
Enhanced noise modeling improves the agreement between data and predictions for a gravitational-wave background from coalescing supermassive black hole binaries.
For the past two decades, Pulsar Timing Arrays (PTAs) have regularly observed many millisecond pulsars with the aim of detecting and characterizing gravitational waves in the nanohertz frequency band. Since 2020, different PTA collaborations have reported growing evidence for a stochastic gravitational-wave background. A key element of these measurements is understanding and modeling systematic errors and noise contamination. An international team led by researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hannover, Germany, developed an improved noise model to analyze data from the European Pulsar Timing Array. By addressing known limitations, such as modeling the distribution of pulsar-intrinsic noise across the population, epoch-correlated noise, and transient noise in a particular pulsar, and by using noise model averaging instead of model selection, the researchers have mitigated a tension between earlier analyses of the data and the theoretical predictions. The background’s spectral index value inferred by the improved analysis is more consistent with the value predicted for a gravitational-wave background from coalescing black hole binaries in circular orbits. Additionally, the inferred amplitude of the background is in less tension with theoretical expectations.
Paper abstract
Constraining the origin of the nanohertz gravitational-wave background necessitates precise noise modelling to avoid parameter estimation biases. In this work, we find the inferred properties of the putative gravitational wave background in the second data release of the European Pulsar Timing Array to be in better agreement with theoretical expectations under the improved noise model. In particular, our improved noise models show consistency of the background's strain spectral index with the value of -2/3, favoring the population of supermassive black hole binaries as the origin of the background. Our results further suggest that the observed gravitational wave emission is the dominant source of the binary energy loss, with no evidence of environmental effects or eccentric orbits. At the reference gravitational wave frequency of yr-1 we also find a lower power-law strain amplitude of the background than in previous data analyses. This mitigates some of the tensions of the strain amplitude with the expected number density and mass scale of binaries discussed in the literature. Our analysis demonstrates the importance of accurate modelling of radio pulsar pulse profile variations, hierarchical properties of noise across pulsars, as well as noise model averaging, when inferring properties of the gravitational wave background.
