Constraints on Axions and Axionlike Particles from Fermi Large Area Telescope Observations of Neutron Stars

We present constraints on the nature of axions and axionlike particles (ALPs) by analyzing gamma--ray data from neutron stars using the Fermi Large Area Telescope. In addition to axions solving the strong CP problem of particle physics, axions and ALPs are also possible dark matter candidates. We investigate axions and ALPs produced by nucleon--nucleon bremsstrahlung within neutron stars. We derive a phenomenological model for the gamma--ray spectrum arising from subsequent axion decays. By analyzing 5 years of gamma-ray data (between 60 MeV and 200 MeV) for a sample of 4 nearby neutron stars, we do not find evidence for an axion or ALP signal, thus we obtain a combined 95\% confidence level upper limit on the axion mass of 7.9$\times 10^{-2}$ eV, which corresponds to a lower limit for the Peccei-Quinn scale $f_a$ of 7.6$\times 10^7$ GeV. Our constraints are more stringent than previous results probing the same physical process, and are competitive with results probing axions and ALPs by different mechanisms.