Probing Primordial Black Hole Mergers in Clusters with Pulsar Timing Data

TL;DR

This paper investigates whether primordial black hole mergers could explain the gravitational wave background detected by pulsar timing arrays, finding that standard models are unlikely to account for the observations, which favor astrophysical sources.

Contribution

The study performs a Bayesian analysis of PTA data considering broad PBH mass distributions and constraints, showing standard PBH models are insufficient to explain the GW background.

Findings

Scalar-induced GWs dominate the nHz range.

Standard PBH merger models cannot explain the observed signal.

Astrophysical sources are strongly favored over PBH models.

Abstract

We consider the possibility that the stochastic gravitational wave (GW) background suggested by Pulsar Timing Array (PTA) datasets is sourced by Primordial Black Holes (PBHs). Specifically, we perform a Bayesian search in the International PTA Data Release 2 (IPTA DR2) for a combined GW background arising from scalar perturbations and unresolved PBH mergers, assuming a broad PBH mass distribution. In our analysis, we incorporate constraints on the curvature power spectrum from CMB $\mu$-distortions and the overproduction of PBHs, which significantly suppress the contribution of PBH mergers to the total GW background. We find that scalar-induced GWs dominate the nHz frequency range, while PBH mergers alone cannot account for the observed signal under the standard PBH formation scenario involving Gaussian perturbations, and including only Poissonian PBH clustering. However, specific PBH models, such as those with enhanced clustering, could yield a GW background dominated by PBH mergers. Overall, we find that the IPTA DR2 strongly favors an astrophysical origin for the reported common-spectrum process over the PBH models considered in this analysis.