Fermi Gamma-ray Space Telescope

Constraints on Axions from Spatially-Extended Gamma Ray Emission from Neutron Stars

Bijan Berenji


Axions are hypothetical particles proposed to solve the strong CP problem in QCD and may constitute a significant fraction of the dark matter in the Universe. Axions are expected to be produced in neutron stars and subsequently decay, producing gamma-rays detectable by the Fermi Large Area Telescope (Fermi-LAT). Light QCD axions with masses < 1eV may travel a long range before they decay into gamma rays, rendering neutron stars as potentially spatially extended gamma-ray sources. We investigate the spatial emission of gamma rays using phenomenological models of neutron star axion emission. We consider projected sensitivities for mass limits on axions from J0108-1431, a neutron star at a distance of 130 pc. Based on the extended angular profile of the source, the expected sensitivity of the 95% CL upper limit on the axion mass from J0108-1431 is 0.76 meV for an inner temperature of the neutron star of 20 MeV.