The Gravitational-wave Background Null Hypothesis: Characterizing Noise in Millisecond Pulsar Arrival Times with the Parkes Pulsar Timing Array

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 γ = 13/3, the value predicted for a stochastic GW background from a population of supermassive black hole binaries driven solely by GW emission.

NANOGrav results and dark first order phase transitions

“…In addition to previous analyses [6][7][8][9], the recent NANOGrav 15-year data also reports the evidence for quadrupolar correlations that follow the signature described by Hellings and Downs [10] and is then conclusive with regard to the nature of the detection. Similar evidences have been independently reported by the European Pulsar Timing Array (EPTA) [11][12][13][14][15][16], the Parkes Pulsar Timing Array (PPTA) [17][18][19], and the Chinese Pulsar Timing Array (CPTA) [20], pointing at a broadly consistent picture.…”

Inflationary interpretation of the stochastic gravitational wave background signal detected by pulsar timing array experiments

“…A number of pulsar timing array (PTA) experiments (NANOGrav, EPTA+InPTA, PPTA, and CPTA) have recently confirmed the detection of a common-spectrum low-frequency stochastic signal across the pulsars in their latest datasets, with the additional compelling evidence of Hellings-Downs inter-pulsar correlations, achieving the first convincing detection of a stochastic gravitational wave background (SGWB) in the nHz frequency range [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. This has officially broadened the horizons of GW astronomy, and the quest is now open for determining the origin of these signals.…”

“…Very recently, in June 2023, various PTA experiments, including NANOGrav, EPTA, PPTA, and the Chinese Pulsar Timing Array (CPTA), in the case of EPTA including also data from the Indian PTA (InPTA), reported on the analyses of their latest datasets, which all confirm the presence of excess red common-spectrum signals, with strain amplitude of order O(10 −15 ) at the reference frequency f = 1 yr −1 [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Importantly, all analyses report evidence (with varying strength) for HD correlations, which point to a genuine GW origin for the signals, in turn making these the first convincing detections of a SGWB signal in the nHz range.…”

“…In June 2023 the NANOGrav, EPTA, PPTA, and CPTA PTA experiments reported evidence for excess red common-spectrum signals in their latest datasets (NANOGrav 15-year, EPTA DR2 complemented with InPTA DR1, PPTA DR3, and CPTA DR1), with interpulsar correlations following the HD pattern [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Considering that all four signals are consistent with each other, and the fact that NANOGrav achieved the most precise determination of the spectral index (which not all experiments managed to infer), for concreteness and simplicity I will focus on the NANOGrav 15-year signal in the remainder of this work, but I expect my main conclusions to broadly carry over to the results of the other three PTA experiments as well.…”

Eccentric Minidisks in Accreting Binaries

“…These objects are powerful sources of low-frequency gravitational wave (GW) radiation, and their population has long been theorized to generate a stochastic GW background (Carr 1980;Rajagopal & Romani 1995). The recent detection of such a background with pulsar timing arrays (Agazie et al 2023;Antoniadis et al 2023a;Reardon et al 2023a;Xu et al 2023), as well as indications that the SMBHB population is indeed its likely source (Afzal et al 2023;Antoniadis et al 2023b;Reardon et al 2023b), creates a new imperative to identify individual SMBHB systems.…”