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

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 ≪10 keV ) axion-like particles to neutrons. Using Markov-Chain Monte Carlo we find that for the values of axion-neutron coupling g_a^{n n 2}>7.7 ×10^-20 (90% c.l.) the axion cooling from the bremsstrahlung reaction n +n →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 ×10⁷GeV for KSVZ axions and f_a>1.7 ×10⁹GeV for DFSZ axions. The high temperature of this neutron star also allows us to tighten constraints on the size of the nucleon pairing gaps.