Gamma-ray line emission from the Local Bubble

Abstract

Deep-sea archives that include intermediate-lived radioactive Fe particles suggest the occurrence of several recent supernovae inside the present-day volume of the Local Bubble during the last sim 10\,Myr. The isotope Fe is mainly produced in massive stars and ejected in supernova explosions, which should always result in a sizeable yield of Al from the same objects. Fe and Al decay with lifetimes of 3.82 and 1.05\,Myr, and emit gamma rays at 1332 and 1809\,keV respectively. These gamma rays have been measured as diffuse glow of the Milky Way, and would also be expected from inside the Local Bubble as foreground emission. Based on two scenarios, one employing a geometrical model and the other state-of-the-art hydrodynamics simulations, we estimated the expected fluxes of the 1332 and 1809\,keV gamma -ray lines, as well as the resulting 511\,keV line from positron annihilation due to the Al decay. We find fluxes in the range of $ for all three lines with isotropic contributions of 10--50\,<!PCT!>. We show that these fluxes are within reach for the upcoming COSI-SMEX gamma -ray telescope over its nominal satellite mission duration of 2\,yr. Given the Local Bubble models considered, we conclude that in the case of 10--20\,Myr-old superbubbles, the distributions of Fe and Al are not co-spatial -- an assumption usually made in gamma -ray data analyses. In fact, this should be taken into account however when analysing individual nearby targets for their Fe to Al flux ratio as this gauges the stellar evolution models and the age of the superbubbles. A flux ratio measured for the Local Bubble could further constrain models of Fe deposition on Earth and its moon.