Gamma-ray Emission of 60Fe and 26Al Radioactivities in our Galaxy

Abstract

The isotopes $^{60}$Fe and $^{26}$Al originate from massive stars and their supernovae, reflecting ongoing nucleosynthesis in the Galaxy. We studied the gamma-ray emission from these isotopes at characteristic energies 1173, 1332, and 1809 keV with over 15 years of SPI data, finding a line flux in $^{60}$Fe combined lines of $(0.31\pm 0.06) \times 10^{-3}$ ph cm$^{-2}$ s$^{-1}$ and the $^{26}$Al line flux of $(16.8\pm 0.7) \times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ above the background and continuum emission for the whole sky. Based on the exponential-disk grid maps, we characterise the emission extent of $^{26}$Al to find scale parameters $R_0 =7.0^{+1.5}_{-1.0}$ kpc and $z_0=0.8^{+0.3}_{-0.2}$ kpc, however the $^{60}$Fe lines are too weak to spatially constrain the emission. Based on a point source model test across the Galactic plane, the $^{60}$Fe emission would not be consistent with a single strong point source in the Galactic center or somewhere else, providing a hint for a diffuse nature. We carried out comparisons of emission morphology maps using different candidate-source tracers for both $^{26}$Al and $^{60}$Fe emissions, and suggests that the $^{60}$Fe emission is more likely to be concentrated towards the Galactic plane. We determine the $^{60}$Fe/$^{26}$Al $\gamma$-ray flux ratio at $(18.4\pm4.2)\,\%$ , when using a parameterized spatial morphology model. Across the range of plausible morphologies, it appears possible that $^{26}$Al and $^{60}$Fe are distributed differently in the Galaxy. Using the best fitting maps for each of the elements, we constrain flux ratios in the range 0.2--0.4. We discuss its implications for massive star models and their nucleosynthesis.