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 $R_0=7.0_{-1.0}^{+1.5}$ kpc and $z_0=0.8_{-0.2}^{+0.3}$ 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 ${\hat{I}}^3$-ray flux ratio at $(18.4\pm 4.2)\%$ , 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.

29 pages, 13 figures, 2 table, accept for the publication in ApJ