Axion Gamma-Ray Signatures from Quark Matter in Neutron Stars

We present a theoretical model for detecting axions from neutron stars in a QCD phase of quark matter, in which the gamma-rays are produced from decaying axions. The axions would be produced from a quark-antiquark pair uu̅ or dd̅. The chiral anomaly of QCD and the spontaneously broken symmetry are invoked to explain the non-conservation of the axion current. The axion quark-antiquark coupling can be computed via the Goldberger-Treiman relation, relying on the two-loop coupling form factors from the QCD axion current. From the coupling form factors, the axion emissivities ϵ_a can be derived, from which fluxes can be determined. A Monte Carlo simulation is used to compute the emissivity for axions from said neutron stars, from which the predicted gamma-ray signal from the Fermi Large Area Telescope (Fermi LAT) can be computed. We predict a photon flux which may be detectable by Fermi LAT, and limits on the QCD mass m_a. A signal from LIGO GWS 170817 could be produced from the neutron star merger as well, which is compared to the expected signal from the Fermi LAT. An upper bound m_a≲ 10^-10eV is thereby placed. Very stringent limits on the axion mass of nearly m_a ≃ 10^-14eV could be placed using this model with the closest neutron stars near 100 kpc. Axions could thus be detectable in gamma-rays for neutron stars as distant as 1 Mpc.