Axion cooling of neutron stars. II. Beyond hadronic axions

We study the axion cooling of neutron stars within the Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model, which allows for tree-level coupling of electrons to the axion and locks the Peccei-Quinn charges of fermions via an angle parameter. This extends our previous study [Phys. Rev. D 93, 065044 (2016)] limited to hadronic models of axions. We explore the two-dimensional space of axion parameters within the DFSZ model by comparing the theoretical cooling models with the surface temperatures of a few stars with measured surface temperatures. It is found that axions masses ma = 0.06 to 0.12 eV can be excluded by x-ray observations of thermal emission of neutron stars (in particular by those of Cas A), the precise limiting value depending on the angle parameter of the DFSZ model. It is also found that axion emission by electron bremsstrahlung in neutron star crusts is negligible except for the special case where neutron Peccei-Quinn charge is small enough, so that the coupling of neutrons to axions can be neglected.