Axion-like-particles (ALPs) emitted from the core of a magnetar can convert to photons in its magnetosphere. The resulting photon flux is sensitive to the product of (i) the ALP-nucleon coupling Gan which controls the production cross section in the core and (ii) the ALP-photon coupling gaγγ which controls the conversion in the magnetosphere. We study such emissions in the soft-gamma-ray range (300 keV to 1 MeV), where the ALP spectrum peaks and astrophysical backgrounds from resonant Compton up scattering are expected to be suppressed. Using published quiescent soft-gamma-ray flux upper limits in five magnetars obtained with CGRO COMPTEL and INTEGRAL SPI/IBIS/ISGRI, we put limits on the product of the ALP-nucleon and ALP-photon couplings. We also provide a detailed study of the dependence of our results on the magnetar core temperature. We further show projections of our result for future Fermi-GBM observations. Our results motivate a program of studying quiescent soft-gamma-ray emission from magnetars with the Fermi-GBM.
Lloyd, S. J., Chadwick, P. M., Brown, A. M., Guo, H., & Sinha, K. (2021). Axion constraints from quiescent soft gamma-ray emission from magnetars. Physical Review D, 103(2), Article 023010. https://doi.org/10.1103/physrevd.103.023010