Axion mass limit from observations of the neutron star in Cassiopeia A

TL;DR

This paper uses observations of the neutron star in Cassiopeia A to set constraints on axion-neutron interactions by analyzing its cooling behavior and excess energy losses, providing new limits on axion properties.

Contribution

It introduces a method to estimate axion-neutron coupling strength based on neutron star cooling data, linking astrophysical observations to particle physics constraints.

Findings

Axion-neutron coupling strength estimated as $c_{n}^{2}m_{a}^{2}\,\sim 5.7\times 10^{-6}$ eV$^2$.

Energy losses in the neutron star are approximately twice those explained by neutrino emission alone.

Constraints on the axion mass and coupling are derived from cooling data and superfluid core emission considerations.

Abstract

Direct Chandra observations of a surface temperature of isolated neutron star in Cassiopeia A (Cas A NS) and its cooling scenario which has been recently simultaneously suggested by several scientific teams put stringent constraints on poorly known properties of the superfluid neutron star core. It was found also that the thermal energy losses from Cas A NS are approximately twice more intensive than it can be explained by the neutrino emission. We use these unique data and well-defined cooling scenario to estimate the strength of KSVZ axion interactions with neutrons. We speculate that enlarged energy losses occur owing to emission of axions from superfluid core of the neutron star. If the axion and neutrino losses are comparable we find $c_{n}^{2}m_{a}^{2}\sim 5.7\times 10^{-6}\,\text{eV}^2$, where $m_{a}$ is the axion mass, and $c_{n}$ is the effective Peccei-Quinn charge of the neutron. (Given the QCD uncertainties of the hadronic axion models, the dimensionless constant $c_{n}$ could range from $-0.05$ to $ 0.14$.)