Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array (2306.16215v1)

Abstract: Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which spans 18 years. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise process. Represented as a strain spectrum $h_c = A(f/1 {\rm yr}^{-1})^{\alpha}$, we measure $A=3.1^{+1.3}_{-0.9} \times 10^{-15}$ and $\alpha=-0.45 \pm 0.20$ respectively (median and 68% credible interval). For a spectral index of $\alpha=-2/3$, corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we recover an amplitude of $A=2.04^{+0.25}_{-0.22} \times 10^{-15}$. However, we demonstrate that the apparent signal strength is time-dependent, as the first half of our data set can be used to place an upper limit on $A$ that is in tension with the inferred common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing pulsar positions on the sky. For a process with $\alpha=-2/3$, we measure spatial correlations consistent with a GWB, with an estimated false-alarm probability of $p \lesssim 0.02$ (approx. $2\sigma$). The long timing baselines of the PPTA and the access to southern pulsars will continue to play an important role in the International Pulsar Timing Array.

• The paper employs Bayesian inference on 18 years of data from 30 millisecond pulsars to detect a common-spectrum noise signal with strain parameters A ≈ 3.1×10⁻¹⁵ and α ≈ -0.45.
• The paper finds temporal variability, as the first half of the dataset yields a conflicting upper limit compared to the complete analysis, suggesting nonstationarity in the observed signal.
• The paper’s spatial correlation analysis shows a false-alarm probability of p ≲ 0.02, providing tentative support for an isotropic gravitational-wave background.

Overview of the Search for an Isotropic Gravitational-Wave Background with the Parkes Pulsar Timing Array

The paper entitled "Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array" presents an in-depth investigation using the Parkes Pulsar Timing Array (PPTA) to detect and characterize the stochastic gravitational-wave background (GWB) at nanohertz frequencies. This research relies on data spanning 18 years from 30 millisecond pulsars monitored by the PPTA, a critical part of the global effort to detect gravitational waves (GWs) using pulsar timing arrays (PTAs).

Key Findings and Methodologies

The paper employs Bayesian inference methods to identify and characterize potential GWB signals. The research detects a common-spectrum noise process across the pulsars, represented by a strain spectrum with parameters $A=3.1^{+1.3}_{-0.9} \times 10^{-15}$ and $\alpha=-0.45 \pm 0.20$ (in terms of the strain spectrum model $h_c = A(f/1~{\rm yr}^{-1})^{\alpha}$). Notably, for a spectral index $\alpha = -2/3$, which aligns with the expected contributions from inspiraling supermassive black hole binaries (SMBHBs), the amplitude was estimated at $A = 2.04^{+0.25}_{-0.22} \times 10^{-15}$.

However, the paper reveals that this apparent signal is subject to time dependence. Interestingly, the first half of the dataset produced an upper limit on this amplitude that conflicts with the common-spectrum amplitude inferred from the complete dataset. This points towards the variability or nonstationarity of the process across the data span.

Spatial correlations were explored using hierarchical analysis of pulsar pairs, which confirmed the consistency of observed correlations with those expected from a GWB. The analysis further estimated a false-alarm probability $p \lesssim 0.02$, equating to roughly $2\sigma$ significance, underlining tentative support but not definitive evidence for isotropic GWB.

Implications and Future Directions

The detection of a common-spectrum noise hints at a dominant background, likely due to SMBHBs, yet its time-variable amplitude challenges our understanding of such backgrounds, suggesting potential unmodeled noise sources or errors. The results gained from pulsar timing are vital for enhancing our grasp of such astrophysical phenomena and provide the groundwork for PTAs globally.

Future directions primarily involve reducing uncertainties through longer observational baselines and more pulsars. Collaboration with international PTAs will be crucial in increasing sensitivity, enabling the disambiguation of GWB from other noise sources, such as errors in solar system ephemerides.

Conclusion

This paper significantly contributes to the field by advancing techniques for detecting GWBs and reinforcing the need for vigilance regarding the time-dependent nature of the observed signals. The implications of these findings extend beyond the immediate paper, potentially influencing techniques and analytical methods in future GWB searches globally. Continued observation and methodological refinements are essential for unraveling the precise origin of the observed background noise and confirming GWB detection with high confidence.