Constraining the Axion Mass Through Gamma-Ray Observations of Pulsars

We analyze nine years of PASS 8 Fermi-LAT data in the 60-500 MeV range and determine flux upper limits (ULs) for 17 gamma-ray dark pulsars as a probe of axions produced by nucleon-nucleon Bremsstrahlung in the pulsar core. Using a previously published axion decay gamma-ray photon flux model for pulsars which relies on a high core temperature of 20 MeV, we improve the determination of the UL axion mass (m(a)), at 95% confidence level, to 9.6 x 10(-3) MeV, which is a factor of 8 improvement on previous results. We show that the axion emissivity (energy loss rate per volume) at realistic lower pulsar core temperatures of 4 MeV or less is reduced to such an extent that axion emissivity and the gamma-ray signal becomes negligible. We consider an alternative emission model based on energy loss rate per mass to allow m(a) to be constrained with Fermi-LAT observations. This model yields a plausible UL m(a) of 10(-6) eV for pulsar core temperature less than 0.1 MeV, but knowledge of the extent of axion-to-photon conversion in the pulsar B field would be required to make a precise UL axion mass determination. The peak of the axion flux is likely to produce gamma rays in the less than 1 MeV energy range, and so future observations with medium-energy gamma-ray missions, such as AMEGO and e-ASTROGAM, will be vital to further constrain UL m(a).