The origin of antimatter in the Milky Way

• Applicant: Dr. Thomas Siegert
• Subject Area: Astrophysics and Astronomy
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 412997271

The central regions of the Milky Way show a cumulation of antimatter. For more than 40 years, the annihilation radiation of electrons with their anti-particles, the positrons, is observed from the direction of the galactic centre (bulge). This annihilation is measured at photon energies of 511 keV, according to the rest mass of the electron. Since the dawn of gamma-ray astronomy, the questions are now, where the antimatter particles come from, and why only the bulge of our Galaxy appears bright, although the sources are expected to be in the disk of the Milky Way. The production mechanisms for positrons in particle physics are versatile, so that different astrophysical objects must be considered as candidates. Radioactive decays of freshly-synthesised nuclei, such as from massive stars and supernovae, produce positrons with kinetic energies of the order of 1 MeV. In particular 26Al, with a characteristic lifetime of one million years, is a prominent positron producer, because its decay gamma-rays at 1.8 MeV are also observed in the Milky Way. Hence, by coherent measurements, a direct link between production and annihilation can be set up. Alternatively, positrons from pair-production in strong electromagnetic fields around neutron stars or black holes, can reach TeV energies. However, the spectral characteristics of the annihilation radiation, measured with the gamma-ray spectrometer telescope SPI aboard the ESA satellite INTEGRAL, point to very slow, already thermalised positrons. Thus, the propagation and energy losses of the initially relativistic cosmic-ray anti-particles must play an important role in the unsolved “positron-puzzle”. If the positrons move along the galactic magnetic field from the disk towards the bulge, a fraction of them should nevertheless be annihilated on their way, if they experience the same conditions in the interstellar medium. As the only instrument, INTEGRAL/SPI found such a faint signal from the disk. The goal of this project is now to measure the celestial 511 keV radiation complementary with the new and modern Compton Spectrometer and Imager (COSI). COSI observed the gamma-ray sky, between 200 keV and 5 MeV, for 46 days during a balloon flight in 2016. This project includes the first joint data analysis of telescopes which measured the diffuse galactic gamma radiation simultaneously. Such an analysis will help to constrain the morphology of both antimatter sinks as well as antimatter sources, such as 26Al. By spectral analyses, the respective interstellar medium conditions will be determined.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Steven E. Boggs, Ph.D.