The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background

TL;DR

This paper reports a search for an isotropic stochastic gravitational-wave background using 12.5 years of pulsar timing data, finding strong evidence for a common process but no definitive spatial correlation consistent with gravitational waves.

Contribution

The study provides the first Bayesian analysis of a long-term pulsar timing dataset revealing a common-spectrum process with potential astrophysical implications, but no confirmed gravitational wave detection.

Findings

Detected a common-spectrum process with median strain amplitude of 1.92e-15.

Bayes factor exceeds 10,000 favoring a common process over independent noise.

No significant evidence of quadrupolar correlations, thus no confirmed gravitational wave detection.

Abstract

We search for an isotropic stochastic gravitational-wave background (GWB) in the $12.5$-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. Our analysis finds strong evidence of a stochastic process, modeled as a power-law, with common amplitude and spectral slope across pulsars. The Bayesian posterior of the amplitude for an $f^{-2/3}$ power-law spectrum, expressed as the characteristic GW strain, has median $1.92 \times 10^{-15}$ and $5\%$--$95\%$ quantiles of $1.37$--$2.67 \times 10^{-15}$ at a reference frequency of $f_\mathrm{yr} = 1 ~\mathrm{yr}^{-1}$. The Bayes factor in favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds $10,000$. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with general relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature.