Constraints on axion-like particles and nucleon pairing in dense matter from the hot neutron star in HESS J1731-347

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 $\ensuremath{\ll}10\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$) axion-like particles to neutrons. Using Markov-Chain Monte Carlo we find that for the values of axion-neutron coupling ${g}_{ann}^{2}>7.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}$ (90% c.l.) the axion cooling from the bremsstrahlung reaction $n+n\ensuremath{\rightarrow}n+n+a$ is too rapid to account for the high observed surface temperature. This implies that the Pecci-Quinn scale or axion decay constant ${f}_{a}>6.7\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$ for KSVZ axions and ${f}_{a}>1.7\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$ for DFSZ axions. The high temperature of this neutron star also allows us to tighten constraints on the size of the nucleon pairing gaps.