The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals (2306.16214v1)

Abstract: We present the results of the search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies using the second data release of the European Pulsar Timing Array (EPTA) for 25 millisecond pulsars and a combination with the first data release of the Indian Pulsar Timing Array (InPTA). We analysed (i) the full 24.7-year EPTA data set, (ii) its 10.3-year subset based on modern observing systems, (iii) the combination of the full data set with the first data release of the InPTA for ten commonly timed millisecond pulsars, and (iv) the combination of the 10.3-year subset with the InPTA data. These combinations allowed us to probe the contributions of instrumental noise and interstellar propagation effects. With the full data set, we find marginal evidence for a GWB, with a Bayes factor of four and a false alarm probability of $4\%$. With the 10.3-year subset, we report evidence for a GWB, with a Bayes factor of $60$ and a false alarm probability of about $0.1\%$ ($\gtrsim 3\sigma$ significance). The addition of the InPTA data yields results that are broadly consistent with the EPTA-only data sets, with the benefit of better noise modelling. Analyses were performed with different data processing pipelines to test the consistency of the results from independent software packages. The inferred spectrum from the latest EPTA data from new generation observing systems is rather uncertain and in mild tension with the common signal measured in the full data set. However, if the spectral index is fixed at 13/3, the two data sets give a similar amplitude of ($2.5\pm0.7)\times10^{-15}$ at a reference frequency of $1\,{\rm yr}^{-1}$. By continuing our detection efforts as part of the International Pulsar Timing Array (IPTA), we expect to be able to improve the measurement of spatial correlations and better characterise this signal in the coming years.

• The paper introduces a multi-faceted analysis of 24.7 to 25.4 years of pulsar timing data to robustly search for an isotropic gravitational wave background.
• It employs both Bayesian and frequentist methods, achieving a Bayes factor of 60 and a false alarm probability of about 0.1%, indicating near 3σ detection significance.
• The analysis constrains spectral characteristics and cross-correlation models, providing key insights to guide future coordinated pulsar timing array efforts.

Analyzing the European Pulsar Timing Array's Pursuit of Gravitational Waves

The European Pulsar Timing Array's second data release (EPTA DR2) represents a significant advancement in the empirical search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies. By utilizing observations from 25 millisecond pulsars, the EPTA DR2 data set, supplemented by the first data release of the Indian Pulsar Timing Array (InPTA), endeavors to explore the presence of gravitational waves emanating from sources such as supermassive black hole binaries (SMBHBs). The analysis presented in the paper employs both Bayesian and frequentist methods to assess the presence of gravitational waves and to understand the astrophysical implications of the results.

Key Findings and Methodologies

1. Analysis Scope and Data Combinations: The paper explores three primary analysis configurations: the full 24.7-year EPTA data set termed DR2full, a 10.3-year subset with modern observing systems, and DR2full augmented with InPTA data, extending up to 25.4 years. These configurations provide a comprehensive investigation of potential GWB signals while accounting for instrumental noise and interstellar propagation effects.
2. Detection Metrics: The paper introduces a broad Bayesian framework leveraging tools like PTMCMCSAMPLER and ENTERPRISE, complemented by a frequentist optimal statistic (OS) approach. The Bayesian evidence, quantified through parameters such as the Bayes factor, indicates a notable enhancement in the likelihood of GWB detection with the DR2new subset, which yields a Bayes factor of 60 and a false alarm probability of about 0.1%, denoting a significance approaching the 3σ level.
3. Spectral Characteristics and Cross-Correlation Analysis: A key insight from the analysis is the characterization of the common red signal (CRS), the amplitude of which varies between different observational configurations. Fixing the spectral index at 13/3 aligns the amplitude measurements across data sets, offering constraints on this potential GWB. The paper evaluates spatial correlations using various models, notably the Hellings-Downs (HD) curve, and finds discrepancies in angular bins related primarily to noise characteristics in the data.
4. Conceptual Challenges and Next Steps: The paper discusses potential issues including the role of varying spectral index assumptions and instrumental effects that may skew the CRS recovery between the full data set and the recent subset. The onset of a continuous gravitational wave (CGW) alternative is examined, with mixed evidences indicating the need for further investigation. The authors suggest that future data releases and coordinated efforts, such as those with the International Pulsar Timing Array (IPTA), will be crucial to disentangling these complexities.

Implications and Future Directions

The research prompts significant considerations for the astrophysical community: the evidence for a GWB, while suggestive, requires corroboration by extended observational programs and integration with global pulsar timing arrays. The potential detection of a gravitational wave background feeds into cosmological models of SMBH formation and merger histories, particularly concerning the amplitude and spectral index of the observed signals. Moreover, the data pave the path for future exploration into cosmic phenomena potentially originating from the early universe's dense landscape, presenting implications for our understanding of cosmic structure formation under cold dark matter scenarios.

Going forward, resolving the incongruities in the spectral data and improving the characterization of spatial correlations will be paramount. As more data are assimilated with contributions from global arrays like NANOGrav and the Parkes Pulsar Timing Array, the concerted efforts will likely offer richer insights into these cosmic waveforms, potentially unveiling nuanced facets of gravitational wave physics and complementing the ground-based detections that have heralded the era of gravitational astronomy. In parallel, simulations capturing realistic noise attributes and prediction models will bolster the fidelity and interpretability of the observational repertoire in forthcoming pursuits.