Constraining Axion Mass through Gamma-ray Observations of Pulsars
Abstract
We analyze 9 years of PASS 8 Fermi-LAT data in the 60-500 MeV range and determine flux upper limits (UL) 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 (ma), at 95 percent confidence level, to 9.6 × 10-3 eV, 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 ma to be constrained with Fermi-LAT observations. This model yields a plausible UL ma of 10-6 eV for pulsar core temperature < 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 axion flux is likely to produce gamma-rays in the ≤ 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 ma.