Telescope Concepts in Gamma-Ray Astronomy

The Positron Puzzle is a half-century old conundrum about the origin of the Galactic $\gamma $ γ -ray emission line at photon energies of 511 keV, and the shape of its morphology, showing a bulge-to-disk luminosity ratio of $\sim 1$ ∼ 1 – unlike any astrophysical source distribution. Positrons ($\mathrm {e}^{+}\mathrm {s}$ e + s ) that have been cooled to the eV scale capture electrons ($\mathrm {e}^{-}\mathrm {s}$ e − s ) and form the intermediate bound state of Positronium (Ps) which decays on a nano-second timescale into two or three photons. Assuming the emission to originate from the Galactic bulge, centre, and disk, a visible annihilation rate in the Milky Way of $\sim 5 \times 10^{43}\,\mathrm{e^{+}\,s^{-1}}$ ∼ 5 × 10 43 e + s − 1 has to be explained, either by a quasi-steady state of production and annihilation, or by possibly multiple burst-like events that flood the Galaxy with $\mathrm {e}^{+}\mathrm {s}$ e + s , then fading away on a Myr timescale.In this paper, I will review what the real Positron Puzzle is, where data and simulations have been used inadequately which resulted in false claims and an apparent quandary, what we really know and absolutely not know about the topic, and how this epistemic problem might be advancing.

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Gamma-ray line emission from the Local Bubble

Siegert,

Schulreich,

Bauer

et al. 2024

A&A

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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.

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Telescope Concepts in Gamma-Ray Astronomy

Cited by 4 publications

References 130 publications

Germanium Detectors for MeV Gamma-Ray Astrophysics with the Compton Spectrometer and Imager

Germanium Detectors for MeV Gamma-Ray Astrophysics with the Compton Spectrometer and Imager

The Positron Puzzle

Gamma-ray line emission from the Local Bubble

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