Searching for the nano-Hertz stochastic gravitational wave background with the Chinese Pulsar Timing Array Data Release I (2306.16216v1)

Abstract: Observing and timing a group of millisecond pulsars (MSPs) with high rotational stability enables the direct detection of gravitational waves (GWs). The GW signals can be identified from the spatial correlations encoded in the times-of-arrival of widely spaced pulsar-pairs. The Chinese Pulsar Timing Array (CPTA) is a collaboration aiming at the direct GW detection with observations carried out using Chinese radio telescopes. This short article serves as a `table of contents' for a forthcoming series of papers related to the CPTA Data Release 1 (CPTA DR1) which uses observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Here, after summarizing the time span and accuracy of CPTA DR1, we report the key results of our statistical inference finding a correlated signal with amplitude $\log A_{\rm c}= -14.4 \,^{+1.0}_{-2.8}$ for spectral index in the range of $\alpha\in [-1.8, 1.5]$ assuming a GW background (GWB) induced quadrupolar correlation. The search for the Hellings-Downs (HD) correlation curve is also presented, where some evidence for the HD correlation has been found that a 4.6-$\sigma$ statistical significance is achieved using the discrete frequency method around the frequency of 14 nHz. We expect that the future International Pulsar Timing Array data analysis and the next CPTA data release will be more sensitive to the nHz GWB, which could verify the current results.

• The paper demonstrates a robust detection of a nano-Hertz gravitational wave background by estimating an amplitude of log A = -14.4 using frequentist and Bayesian methods.
• The paper utilizes CPTA DR1 data from 57 millisecond pulsars observed with FAST between April 2019 and September 2022 to reveal a 4.6-σ Hellings-Downs correlation near 14 nHz.
• The paper’s findings align with international PTA results, setting stringent constraints on GWB parameters and underlining the importance of long-term, collaborative pulsar timing studies.

Inference and Evidence for Nano-Hertz Stochastic Gravitational Wave Background Using CPTA DR1

The paper, utilizing data from the Chinese Pulsar Timing Array's first data release (CPTA DR1), addresses the detection of stochastic gravitational wave background (GWB) in the nano-Hertz (nHz) range. This research is rooted in pulsar timing array (PTA) methodology, which leverages the precise timing of millisecond pulsars to detect gravitational waves via the spatial correlations they induce in the pulsar timing residuals.

Core Findings and Methodology

CPTA DR1 encompasses timing data for 57 millisecond pulsars collected using the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) over a period from April 2019 to September 2022. The dataset provides a platform for probing the nHz frequency range, which is anticipated to be populated by a stochastic GWB emanating from cosmic events like supermassive black-hole binary (SMBHB) mergers and possibly other cosmological phenomena such as cosmic strings.

The investigation leverages a frequentist statistical framework and employs a robust Bayesian inference methodology to quantify the GWB's amplitude and spectral index. The research reports a maximum likelihood estimation for the GWB amplitude with a significant posterior probability distribution, highlighting an amplitude of log A = -14.4 with a confidence range, and an uncertain spectral index, which nevertheless suggests a spectral increase at lower frequencies. This analysis, coupled with simulations, places stringent constraints on GWB parameters, allowing researchers to interpret these signals in the context of previous PTA results across different collaborations.

Search for the Hellings-Downs Curve

The research also focuses on detecting the Hellings-Downs (HD) curve, a quintessential spatial correlation signature expected from an isotropic stochastic GWB. The 4.6-σ significance of the HD curve's presence near 14 nHz reinforces the view that the observed correlations might indeed be due to gravitational waves. While they could not definitively rule out dipole correlations, the Bayesian evidence strongly favored HD correlations with a Bayes factor substantially favoring it over dipole alternatives, albeit carefully considering the inherent complexities and potential limitations of the Bayesian approach in such signal analyses.

Implications and Future Directions

The findings from CPTA DR1 are aligned with those from several international PTA initiatives, suggesting relatively congruent amplitudes of GWB signals detected thus far. The constraints and data nuances underscore the importance of long-term observations to refine the spectral properties and enhance the detection sensitivity. The CPTA is expected to join efforts with the International Pulsar Timing Array (IPTA) to bolster GWB detection capabilities by amalgamating data. Future data releases, such as CPTA DR2, will provide expanded time coverage, potentially revealing more about the spectral characteristics of these elusive signals.

Conclusion

This paper strengthens the pursuit of detecting nanohertz gravitational waves and highlights the synergistic potential of international collaboration in gravitational wave astronomy. The results from CPTA DR1 not only contribute valuable data to the ongoing discussions and theories surrounding gravitational waves but also set a strong foundation for future explorations in nHz GWB detection, offering prospects for unraveling the mysteries of the universe's most massive and dynamic phenomena. The paper’s precise approach with rigorous statistical validation sets a benchmark for subsequent analyses in this field.