Gamma Ray Line Emission from decaying Al-26 inside the Local Bubble
Niklas Bauer, Rudi Reinhardt, Thomas Siegert und Saurabh Mittal
University of Wuerzburg
Context: The 1809 keV gamma-ray line of the Milky Way shows the abundance of Al-26 in the Galaxy. During the decay of Al-26 positrons are produced, eventually ending in another emission line while annihilating at 511 keV. Both lines have been observed by COMPTEL (1809 keV) and INTEGRAL/SPI (511 keV) and suggest some large-scale foreground. Aims: We are looking for an isotropic foreground for these two gamma-ray lines that could be linked to the Local Bubble and if this could be seen with the Compton Spectrometer and Imager, COSI, which is planned to launch in 2027. Methods: A model based on line-of-sight integration is developed and the radioactive decay is translated into an emissivity profile for the primary (1809 keV) and secondary (511 keV) photons. Taking into account the lookback time from two presumed supernovae, we obtain all-sky emission maps for both emission lines. The resulting maps are then used to estimate the detectability with COSI using Monte Carlo simulations including a simplified mass model. Results: From line-of-sight integration and for different positions of two supernovae model expected flux for Al-26 we find a total integrated all-sky 1809 keV line flux F_1809 = (0.73 − 6.4) · 10^(−6) ph /cm^2 /s with an isotropic fraction of 50%. The total integrated all-sky 511 keV line flux is F_511 = (0.49 − 1.5) · 10^(−5) ph /cm^2 /s with an isotropic fraction of 65%. Conclusions: At high Galactic latitudes, the chances to observe gamma-ray line emission from the Local Bubble are within reach for the planned 2-year COSI-SMEX mission.