The gravitational-wave background null hypothesis: Characterizing noise in millisecond pulsar arrival times with the Parkes Pulsar Timing Array

TL;DR

This paper develops detailed noise models for millisecond pulsar timing data to accurately characterize the noise environment, which is essential for detecting or constraining the gravitational-wave background in pulsar timing arrays.

Contribution

It introduces comprehensive noise models for MSPs in the PPTA, enabling more accurate null hypothesis formulation for gravitational wave searches.

Findings

Noise models significantly influence the properties of the inferred common-spectrum process.

The recovered common-spectrum noise aligns with the expected signature of a gravitational-wave background.

Validated models show the noise is consistent with Gaussianity and proper residual correlations.

Abstract

The noise in millisecond pulsar (MSP) timing data can include contributions from observing instruments, the interstellar medium, the solar wind, solar system ephemeris errors, and the pulsars themselves. The noise environment must be accurately characterized in order to form the null hypothesis from which signal models can be compared, including the signature induced by nanohertz-frequency gravitational waves (GWs). Here we describe the noise models developed for each of the MSPs in the Parkes Pulsar Timing Array (PPTA) third data release, which have been used as the basis of a search for the isotropic stochastic GW background. We model pulsar spin noise, dispersion measure variations, scattering variations, events in the pulsar magnetospheres, solar wind variability, and instrumental effects. We also search for new timing model parameters and detected Shapiro delays in PSR~J0614$-$3329 and PSR~J1902$-$5105. The noise and timing models are validated by testing the normalized and whitened timing residuals for Gaussianity and residual correlations with time. We demonstrate that the choice of noise models significantly affects the inferred properties of a common-spectrum process. Using our detailed models, the recovered common-spectrum noise in the PPTA is consistent with a power law with a spectral index of $\gamma=13/3$, the value predicted for a stochastic GW background from a population of supermassive black hole binaries driven solely by GW emission.