Gamma-ray line emission from the Local Bubble

Abstract

Deep-sea archives that include intermediate-lived radioactive ${}^{60}\mathrm{Fe}$ 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 ${}^{60}\mathrm{Fe}$ is mainly produced in massive stars and ejected in supernova explosions, which should always result in a sizeable yield of ${}^{26}\mathrm{Al}$ from the same objects. ${}^{60}\mathrm{Fe}$ and ${}^{26}\mathrm{Al}$ 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 estimate 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 ${}^{26}\mathrm{Al}$ ${\beta }^{+}$-decay. We find fluxes in the range of $10^{-6}$-$10^{-5}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ for all three lines with isotropic contributions of 10-50%. We show that these fluxes are within reach for the upcoming COSI-SMEX $\gamma$-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 ${}^{60}\mathrm{Fe}$ and ${}^{26}\mathrm{Al}$ 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 ${}^{60}\mathrm{Fe}$ to ${}^{26}\mathrm{Al}$ 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 ${}^{60}\mathrm{Fe}$ deposition on Earth and its moon.

accepted in A&A, 18 pages, 13 figures, 2 tables

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• Imaging the positron annihilation line with 20 years of INTEGRAL/SPI observations