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Acero, F., Ackermann, M., Ajello, M., Albert, A., Atwood, W., Axelsson, M., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., et al. (2015) Fermi Large Area Telescope Third Source Catalog. The Astrophysical Journal Supplement Series, 218, 23-64.

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• TITLE: Constraints on Axions from a Relativistic Model of Spatially Extended Gamma-Ray Emission from Neutron Stars

  AUTHORS: Bijan Berenji

  KEYWORDS: Axions, Particle Astrophysics, Dark Matter

  JOURNAL NAME: Journal of Modern Physics, Vol.15 No.11, October 31, 2024

  ABSTRACT: 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 superfluid neutron stars and subsequently decay, producing gamma-rays detectable by the Fermi Large Area Telescope (Fermi-LAT). Considering that light QCD axions, as opposed to axions > 1 eV, may travel a long range before they decay into gamma rays, neutron stars may appear as a spatially extended source of gamma rays. We extend our previous search for gamma rays from axions, based on a point source model, to consider the neutron star as an extended source of gamma rays. The extended consideration of neutron stars leads to higher sensitivity to searches for axions, as it will be shown. We investigate the spatial emission of gamma rays using phenomenological models of neutron star axion emission. We present models including the fundamental astrophysics and relativistic, extended gamma-ray emission from axions around neutron stars. A Monte Carlo simulation of the LAT gives us an expectation for the extended angular profile and spectrum. For a source of ≃ 100 pc, we predict a mean angular spread of ≃ 2˚ with gamma-ray energies in the range 10 - 200 MeV, due to the cutoff of the spin-structure function S σ ( ω ) . We demonstrate the feasibility of setting more stringent limits for axions in this mass range, excluding a range not probed by observations before. We consider projected sensitivities for mass limits on axions from RX J1856-3754, a neutron star at a distance of 130 pc. The limit based on 7.9 years of Fermi-LAT data is 3.9 meV for an inner temperature of the neutron star of 20 MeV.