The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background (2009.04496v2)

Abstract: We search for an isotropic stochastic gravitational-wave background (GWB) in the $12.5$-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. Our analysis finds strong evidence of a stochastic process, modeled as a power-law, with common amplitude and spectral slope across pulsars. The Bayesian posterior of the amplitude for an $f^{-2/3}$ power-law spectrum, expressed as the characteristic GW strain, has median $1.92 \times 10^{-15}$ and $5\%$--$95\%$ quantiles of $1.37$--$2.67 \times 10^{-15}$ at a reference frequency of $f_\mathrm{yr} = 1 ~\mathrm{yr}^{-1}$. The Bayes factor in favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds $10,000$. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with general relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature.

• The paper presents a detailed analysis of a 12.5-year pulsar timing dataset, revealing a common-spectrum stochastic process with a median gravitational wave strain of approximately 1.92×10⁻¹⁵.
• The paper identifies a lack of statistically significant quadrupolar spatial correlations, indicating that the observed signal does not conclusively prove a gravitational-wave background from supermassive black hole binaries.
• The study emphasizes the impact of Bayesian priors on signal interpretation and highlights the need for extended observations and improved noise modeling to refine future gravitational-wave searches.

The NANOGrav 12.5-Year Data Set: Search for an Isotropic Stochastic Gravitational-Wave Background

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Collaboration has presented a detailed investigation into the presence of an isotropic stochastic gravitational-wave background (GWB) using a comprehensive pulsar timing data set spanning 12.5 years. This paper focuses on identifying a common-spectrum process that could be indicative of gravitational waves permeating the universe, particularly those arising in the nanohertz frequency range from cosmic occurrences such as supermassive black hole binaries.

Key Findings and Analysis

1. Detection of a Stochastic Process: The comprehensive analysis of the data set revealed firm evidence of a stochastic process, modeled with a common power-law amplitude and spectral slope across multiple pulsars. The preferred amplitude of this stochastic process showed a characteristic gravitational wave strain with a median value of $1.92 \times 10^{-15}$, corroborated by a Bayes factor exceeding 10,000 favoring a common-spectrum process over independent noise processes.
2. Lack of Quadrupolar Spatial Correlations: Despite detecting the stochastic process, the analysis did not observe statistically significant quadrupolar correlations consistent with general relativity’s predictions for GWBs. This absence suggests that while the data show strong internal consistency, they cannot definitively assert the presence of a GWB manifesting from general relativistic sources. This decision is reinforced by the lack of monopolar or dipolar correlations that could indicate systematic biases such as timing errors.
3. Signal Interpretation and Bayesian Influences: The signal observed stands beyond many previously reported upper limits on GWB strain. The analysis suggests that this disparity can partly be attributed to Bayesian priors, which influence how the data interpretation on pulsar intrinsic noises and other factors like solar system ephemerides are factored into the analysis.

Implications and Future Directions

The implications of this research are far-reaching. Establishing the presence of a GWB would significantly bolster our understanding of cosmic events like supermassive black hole mergers, offering tangible insights into their frequency and intensity throughout the universe. Moreover, this paper provides critical groundwork for refining pulsar-timing models and highlighting the potential adjustments needed in Bayesian methodologies to avoid overly-conservative constraints on GWB estimations.

Looking ahead, the paper underscores the necessity of extended observation periods and improved systematic noise modeling to enhance the robustness of GW detection claims. The ongoing pursuit of longer datasets and advanced noise models is crucial to refine sensitivity and uphold the validity of these preliminary findings. These efforts remain an integral part of the broader international quests embraced by global collaborations like the IPTA to delineate and authenticate the GWB signal.

Conclusion

This research embodies significant progress in the field of gravitational wave astrophysics. Although it stands just short of asserting a GWB detection, the NANOGrav 12.5-year dataset analysis propounds a promising methodological path forward. Continual advancements in data collection, analytical techniques, and international collaboration will be pivotal to realizing the full potential of pulsar-timing arrays for revealing the stochastic GWB and unlocking further cosmic mysteries.