Constraining Postinflationary Axions with Pulsar Timing Arrays

TL;DR

This paper uses pulsar timing array data to constrain axion-like particles produced after inflation, linking gravitational wave backgrounds from cosmic strings to dark matter and dark radiation properties.

Contribution

It introduces new constraints on axion decay constants and masses by analyzing recent PTA data, connecting gravitational wave signals to axion models and cosmological parameters.

Findings

Constraints on axion decay constant and mass from PTA data.

Similar limits to $N_{eff}$ bounds for light ALPs.

New parameter space regions for heavy ALPs with domain-wall contributions.

Abstract

Models that produce Axion-Like-Particles (ALP) after cosmological inflation due to spontaneous $U(1)$ symmetry breaking also produce cosmic string networks. Those axionic strings lose energy through gravitational wave emission during the whole cosmological history, generating a stochastic background of gravitational waves that spans many decades in frequency. We can therefore constrain the axion decay constant and axion mass from limits on the gravitational wave spectrum and compatibility with dark matter abundance as well as dark radiation. We derive such limits from analyzing the most recent NANOGrav data from Pulsar Timing Arrays (PTA). The limits are similar to the $N_{\rm eff}$ bounds on dark radiation for ALP masses $m_a \lesssim 10^{-22}$ eV. On the other hand, for heavy ALPs with $m_a\gtrsim 0.1$ GeV and $N_{\rm DW}\neq 1$, new regions of parameter space can be probed by PTA data due to the dominant Domain-Wall contribution to the gravitational wave background.