Upper Limit on the QCD Axion Mass from Isolated Neutron Star Cooling

TL;DR

This study uses neutron star cooling data to set an upper limit on the QCD axion mass, constraining it to be less than approximately 16 meV within the KSVZ model, thereby informing axion physics and astrophysics.

Contribution

The paper provides the first astrophysical upper limit on the QCD axion mass based on detailed neutron star cooling simulations and observational data.

Findings

No evidence for axions in the neutron star data.

Constraints on axion mass are set to be less than ~16 meV at 95% confidence.

Inclusion of high-density suppression factors affects previous limits.

Abstract

The quantum chromodynamics (QCD) axion may modify the cooling rates of neutron stars (NSs). The axions are produced within the NS cores from nucleon bremsstrahlung and, when the nucleons are in superfluid states, Cooper pair breaking and formation processes. We show that four of the nearby isolated Magnificent Seven NSs along with PSR J0659 are prime candidates for axion cooling studies because they are coeval, with ages of a few hundred thousand years known from kinematic considerations, and they have well-measured surface luminosities. We compare these data to dedicated NS cooling simulations incorporating axions, profiling over uncertainties related to the equation of state, NS masses, surface compositions, and superfluidity. Our calculations of the axion and neutrino emissivities include high-density suppression factors that also affect SN 1987A and previous NS cooling limits on axions. We find no evidence for axions in the isolated NS data, and within the context of the KSVZ QCD axion model we constrain $m_a \lesssim 16$ meV at 95% confidence. An improved understanding of NS cooling and nucleon superfluidity could further improve these limits or lead to the discovery of the axion at weaker couplings.