Searching for Anisotropy in the Gravitational Wave Background Using the Parkes Pulsar Timing Array

TL;DR

This study searches for anisotropic patterns in the nanohertz gravitational wave background using pulsar timing data, employing multiple analysis methods, and finds no significant anisotropy, but highlights the importance of such measurements for understanding GWB origins.

Contribution

First analysis of anisotropy in the nanohertz GWB using Parkes Pulsar Timing Array data with multiple methods, setting the stage for future detailed characterization.

Findings

No statistically significant anisotropy detected.

Identified a hotspot likely due to noise fluctuations.

Highlights the potential of angular power spectrum measurements.

Abstract

In recent years, several pulsar timing array collaborations have reported evidence for a nanohertz gravitational wave background (GWB). Such a background signal could be produced by supermassive binary black holes, early-Universe processes such as inflation and phase transitions, or a mixture of both. One way to disentangle different contributions to the GWB is to search for anisotropic signatures. In this work, we search for anisotropy in the GWB using the third data release of the Parkes Pulsar Timing Array. Our analysis employs both the radiometer method and the spherical harmonic basis to characterize the distribution of GWB power across the sky. We calculate the angular power in the lowest five frequency bins and compare it with detection thresholds determined under the null hypothesis of isotropy. In the 5.26 nHz frequency bin, we identify a hotspot in the reconstructed sky map with a $p$-value of $0.016$ (the lowest in our analysis), which we attribute to noise fluctuations. While our search reveals no statistically significant anisotropy, we expect that the precise measurement of angular power spectrum of the GWB will become instrumental in determining the origin of the nanohertz GWB signal.