Effects of Lighter-than-QCD Axions on Neutron Star Tidal Deformability

TL;DR

This paper investigates how lighter-than-QCD axions influence neutron star properties and tidal deformability, revealing potential for gravitational-wave observations to probe new physics beyond the standard model.

Contribution

It provides the first numerical analysis of the complete dynamics of lighter-than-QCD axions in neutron stars, including low-mass axions with kilometer-scale wavelengths.

Findings

Axion fields significantly affect neutron star mass, radius, and compactness.

Universal tidal deformability-compactness relations are broken by axion effects.

Potential to detect axion signatures through future gravitational-wave observations.

Abstract

Finite density corrections to the lighter-than-QCD axion can invert the effective axion potential, sourcing a non-trivial axion field inside dense objects. We perform the first numerical study of the complete dynamics of the lighter-than-QCD axion in a neutron star in 1+1 general relativity, extending the region of analysis to low-mass axions with kilometer-scale Compton wavelengths. We calculate gravitational effects of the axion field on the neutron star and show that for a broad range of axion masses and decay constants, neutron star properties, such as the mass, radius, and compactness, are affected at the order-1 level. This result indicates that approximate universal tidal deformability-compactness relation for neutron stars is non-trivially broken and can serve as a probe of lighter-than-QCD axions, independent of the unknown nuclear equation of state. We comment on the potential for axion studies with future gravitational-wave observations of neutron stars and applications of this work to other new physics signatures.