Axion Constraints from Quiescent Soft Gamma-ray Emission from Magnetars

Axion-like-particles (ALPs) emitted from the core of magnetars can convert to photons in the strong magnetic field of the magnetosphere. We study such emissions in the soft gamma-ray range from 300 keV to 1 MeV, where the ALP spectrum peaks and astrophysical backgrounds from resonant Compton upscattering are expected to be suppressed. Using published quiescent soft-gamma flux upper limits in 6 Magnetars obtained with $\textit{CGRO}$ COMPTEL, $\textit{INTEGRAL}$ SPI/IBIS/ISGRI and the $\textit{Fermi}$ Gamma Ray Burst Monitor (GBM), we put limits on the ALP-photon coupling obtained from conversions, assuming that ALP emission from the core is just sub-dominant to bounds from neutrino cooling. For core temperatures of $10^9$ K, the constraints on the ALP-photon coupling coming from magnetars 1E 2259+586 and J170849.0-400910 are better than the current limits obtained from the CAST experiment. We provide a detailed study of the dependence of our results on the magnetar core temperature. Our results motivate a program of studying quiescent soft gamma ray emission from magnetars in the 300 keV - 1 MeV band with the $\textit{Fermi}$-GBM.