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Pulsar Timing Array in the past decade

Kuo Liu, Siyuan Chen

TL;DR

The paper surveys the last decade of pulsar timing array (PTA) progress toward nanohertz gravitational waves, detailing instrumentation, data analysis, and the evolving GW signal landscape. It explains the PTA detector framework, including noise modelling, the common red signal, and the Hellings–Downs correlation that provides a smoking-gun signature of a gravitational-wave background, along with continuous waves and memory signals. The review highlights the transition from upper limits to detection of a GW-like common red signal in 2020–2021 and robust evidence for GW origin with HD correlations by 2023–2024, and discusses astrophysical SMBHBs, cosmological/exotic sources, and tests of General Relativity. It concludes with a forward-looking perspective on IPTA DR3, the SKA era, and the potential for multi-messenger discoveries and fundamental physics constraints in the nanohertz GW band.

Abstract

The past decade has been a transformative period for pulsar timing arrays (PTAs) and their search for nanohertz gravitational waves (GWs). This progress has been driven by collective advances in instrumentation for pulsar timing observations, increasingly sophisticated data-analysis techniques, and improved theoretical understanding of the origins of nanohertz GW signals. PTA sensitivity has steadily improved, leading first to progressively more stringent upper limits on the gravitational-wave background (GWB), and subsequently to the identification of a common red-noise process in pulsar timing data, the first hint of a GWB. In 2023, multiple PTA collaborations reported evidence for the Hellings-Downs correlation, widely regarded as the definitive signature of a GWB. These developments place PTAs on the threshold of a confident GW detection and the opening of a new low-frequency window on the GW Universe. In this article, we present an overview of PTA experiments, with particular emphasis on the rapid progress achieved during this pivotal period for PTA and nanohertz GW science.

Pulsar Timing Array in the past decade

TL;DR

The paper surveys the last decade of pulsar timing array (PTA) progress toward nanohertz gravitational waves, detailing instrumentation, data analysis, and the evolving GW signal landscape. It explains the PTA detector framework, including noise modelling, the common red signal, and the Hellings–Downs correlation that provides a smoking-gun signature of a gravitational-wave background, along with continuous waves and memory signals. The review highlights the transition from upper limits to detection of a GW-like common red signal in 2020–2021 and robust evidence for GW origin with HD correlations by 2023–2024, and discusses astrophysical SMBHBs, cosmological/exotic sources, and tests of General Relativity. It concludes with a forward-looking perspective on IPTA DR3, the SKA era, and the potential for multi-messenger discoveries and fundamental physics constraints in the nanohertz GW band.

Abstract

The past decade has been a transformative period for pulsar timing arrays (PTAs) and their search for nanohertz gravitational waves (GWs). This progress has been driven by collective advances in instrumentation for pulsar timing observations, increasingly sophisticated data-analysis techniques, and improved theoretical understanding of the origins of nanohertz GW signals. PTA sensitivity has steadily improved, leading first to progressively more stringent upper limits on the gravitational-wave background (GWB), and subsequently to the identification of a common red-noise process in pulsar timing data, the first hint of a GWB. In 2023, multiple PTA collaborations reported evidence for the Hellings-Downs correlation, widely regarded as the definitive signature of a GWB. These developments place PTAs on the threshold of a confident GW detection and the opening of a new low-frequency window on the GW Universe. In this article, we present an overview of PTA experiments, with particular emphasis on the rapid progress achieved during this pivotal period for PTA and nanohertz GW science.
Paper Structure (37 sections, 34 equations, 13 figures, 1 table)

This paper contains 37 sections, 34 equations, 13 figures, 1 table.

Table of Contents

  1. Introduction
  2. Pulsar Timing
  3. Data acquisition of pulsar timing observation
  4. TOA creation
  5. The deterministic timing model
  6. Noise characterization in pulsar timing
  7. White noise sources
  8. Chromatic red noise sources
  9. Achromatic red noise sources
  10. Correlated noise sources
  11. Noise modelling approaches
  12. The PTA detector
  13. Basic properties of the detector
  14. Complete signal model of the detector
  15. Detecting a gravitational wave background
  16. ...and 22 more sections

Figures (13)

  • Figure 1: Sketch showing the overall data processing chain and products of each stage, from a typical contemporary PTA experiment.
  • Figure 2: Timing residuals of PSR J1909$-$3744 from 15-yr timing observations, before and after subtracting the red noise present in the dataset. The figure is reproduced from lgi+20.
  • Figure 3: Marginalized posterior distributions for the noise parameters (spectral amplitude and index) of both DM and achromatic red noise for PSR J1909$-$3744. These measurements were derived from three different versions of the EPTA dataset, where the EPTA DR2 provides significantly improved frequency coverage in the timing data compared to DR1 EPTA+2023a. The figure is reproduced from EPTA+2023b.
  • Figure 4: Sensitivity curves for the EPTA DR2, PPTA DR3 and NANOGrav 15yr data sets from the IPTA comparison 2024ApJ...966..105A
  • Figure 5: Evolution of the constraints, upper limits and measurements, on the GWB amplitude at $\gamma_{\rm GWB}=13/3$ over the last two deacdes, figure adapted from 2024ResPh..6107719V. The upper limit from the Gamma-ray PTA using Fermi-LAT data 2022Sci...376..521F and the constraint from the MeerKAT PTA meerkat_gwb have also been added.
  • ...and 8 more figures