Constraints on Axion-like Particles and Nucleon Pairing in Dense Matter from the Hot Neutron Star in HESS J1731-347

TL;DR

This study uses the thermal evolution of a young neutron star to set new limits on axion-like particle interactions with neutrons and to refine constraints on nucleon pairing gaps in dense matter.

Contribution

It provides the first astrophysical constraints on axion-neutron coupling from a hot neutron star and tightens bounds on nucleon pairing gaps in dense nuclear matter.

Findings

Axion-neutron coupling $g_{ann}^2 > 7.7 imes 10^{-20}$ is excluded at 90% confidence.

Pecci-Quinn scale $f_a > 6.7 imes 10^7$ GeV for KSVZ axions.

Constraints on nucleon pairing gaps are improved based on neutron star temperature data.

Abstract

If the thermal evolution of the hot young neutron star in the supernova remnant HESS J1731-347 is driven by neutrino emission, it provides a stringent constraint on the coupling of light (mass $\ll 10$ keV) axion-like particles to neutrons. Using Markov-Chain Monte Carlo we find that for the values of axion-neutron coupling $g_{ann}^2 > 7.7 \times 10^{-20}$ (90% c.l.) the axion cooling from the bremsstrahlung reaction $n+n\rightarrow n+n +a$ is too rapid to account for the high observed surface temperature. This implies that the Pecci-Quinn scale or axion decay constant $f_a > 6.7 \times 10^7$ GeV for KSVZ axions and $f_a > 1.7 \times 10^9$ GeV for DFSZ axions. The high temperature of this neutron star also allows us to tighten constraints on the size of the nucleon pairing gaps.