Gamma-Ray Emission of ⁶⁰Fe and ²⁶Al Radioactivity 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 ± 0.06) × 10 −3 ph cm −2 s −1 and the 26 Al line flux of (16.8 ± 0.7) × 10 −4 ph cm −2 s −1 above the background and continuum emission for the whole sky. Based on the exponentialdisk 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 γ-ray flux ratio at (18.4 ± 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.

Galactic 26Al traces metal loss through hot chimneys

“…The non-detection of 26 Al would constrain the 60 Fe/ 26 Al ratio for this supernova to > 0.18, well consistent with expectations of ≈ 2 (Austin et al 2017). INTEGRAL observations of the interstellar medium find 60 Fe/ 26 Al=0.2-0.4 (Wang et al 2020), which suggests a significant or even dominant non-supernova contribution.…”

The GALAH survey: Chemical homogeneity of the Orion complex

“…Models of SN and stellar wind feedbacks by Voss et al (2010) suggest that 26 Al was produced by a few recent supernovae. 60 Fe is another radioactive tracer of SN (Wang et al 2020), which has not been explored yet in the Orion complex. More recent studies of the formation of the Orion complex are based on new Gaia distances and 3D velocities and offer a compelling picture.…”

Steady-state nucleosynthesis throughout the Galaxy

“…A new attempt has been made, using all available accumulated INTE-GRAL data, and exploring results for a wide range of potential morphologies of 60 Fe emission [104]. Here, within a broad band of data from 0.8 to 2 MeV, several continuum bands and the lines from 60 Fe and from 26 Al were fitted consistently to the same adopted morphology.…”