Gamma-ray line emission from the Local Bubble

¹ Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
e-mail: thomas.siegert@uni-wuerzburg.de
² Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
³ RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

Received: 10 April 2024
Accepted: 23 May 2024

Abstract

Deep-sea archives that include intermediate-lived radioactive ⁶⁰Fe particles suggest the occurrence of several recent supernovae inside the present-day volume of the Local Bubble during the last ~10 Myr. The isotope ⁶⁰Fe is mainly produced in massive stars and ejected in supernova explosions, which should always result in a sizeable yield of ²⁶Al from the same objects. ⁶⁰Fe and ²⁶Al decay with lifetimes of 3.82 and 1.05 Myr, and emit γ rays at 1332 and 1809 keV, respectively. These γ 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 γ-ray lines, as well as the resulting 511 keV line from positron annihilation due to the ²⁶Al β⁺ decay. We find fluxes in the range of 10⁻⁶–10⁻⁵ ph cm⁻² s⁻¹ for all three lines with isotropic contributions of 10–50%. We show that these fluxes are within reach for the upcoming COSI-SMEX γ-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 ⁶⁰Fe and²⁶ Al are not co-spatial - an assumption usually made in γ-ray data analyses. In fact, this should be taken into account however when analysing individual nearby targets for their ⁶⁰Fe to²⁶ 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 ⁶⁰Fe deposition on Earth and its moon.