Stochastic gravitational-wave background search using data from five pulsar timing arrays

Abstract

Using public pulse time-of-arrival data from five pulsar timing arrays (PTAs), we search for a stationary, isotropic, and unpolarized nHz stochastic gravitational-wave background (SGWB). This analysis is more sensitive than previous individual PTA searches because the combined 121-pulsar dataset is about four times larger than any single PTA's. For pulsars observed by multiple PTAs, we employ a new "direct combination" method to merge their astrophysical models and data. This avoids the challenge of reconciling different PTA timing models to obtain a single "best" model. A central result of our analysis is the posterior distribution of the amplitude $A_{gw}$ and exponent ${\hat{I}}_{gw}^3$ of the putative SGWB energy-density spectrum, modeled as a power law in frequency. While these results are consistent with a nonzero SGWB amplitude $A_{gw}$, the statistical significance-assessed via a Bayesian odds ratio and noise-marginalized false-alarm probabilities ($p$-values) for three detection statistics-remains below the conventional $5\ddot{I}ƒ$ threshold for a confident detection. The inter-pulsar timing-residual correlation, reconstructed as a function of angle $\hat{I}¸$ between the pulsar lines of sight, matches the Hellings and Downs (HD) prediction.

9 pages, 3 figures plus 5 pages of supplementary material