The origin of antimatter in the Milky Way
Final Report Abstract
In this project, the origin of antimatter in the Milky Way galaxy in the shape of positrons was investigated. When electrons annihilate with their antimatter partners, the positrons, they produce photons at an energy of 511 keV and below. These photons have been measured by various instruments over the last 50 years to originate from the Galactic Centre and Bulge region. The sources of the positrons are unknown and their expected locations differ from the measured annihilation image. With the prototype of the Compton Spectrometer and Imager (COSI), the first of its kind, this conundrum was now investigated with a completely new instrument design: a compact Compton telescope. COSI had a successful 46-day high-altitude balloon flight in 2016. Data from this flight have been analysed in this project. A versatile instrumental background model for COSI's native Compton data space has been developed. The 511 keV emission has been detected by COSI from the Galactic Bulge region with a significance of 7 sigma above the instrumental background due to Earth's gamma-ray albedo. This is the first proof that imaging with a compact Compton telescope works at 511 keV. The emission appears more extended than what has been measured with previous instruments. This is probably due to the fact that Compton telescopes alleviate the design problem of coded-mask telescopes to measure shallow gradients or isotropic emission. A halo component could possibly point to a contribution of dark matter particle annihilation into electron-positron pairs. During the course of this project, an MeV data analysis framework for Compton telescopes, COSIpy, has been developed. COSIpy can handle the raw data, and perform descriptive and inference tasks. It has been applied successfully for the spectral extraction and imaging of the Crab Nebula and Cygnus X-1 up to 1.1 MeV. In addition, the 1.809 MeV emission of decaying Al-26 from massive stars – a possible source of positrons – has been detected thanks to COSIpy. Future avenues of COSI cover a possible satellite mission and are supported by further development of COSIpy. The COSI 2016 balloon flight re-opened the possibility for dark matter to be a major source of Galactic antimatter because the emission appears more extended, independent of the analysis method. A COSI satellite mission, with a possible launch date in 2025, would reveal more possible and exclude several suggested sources, thanks to an improved sensitivity and angular resolution.
Publications
- 2019: Benchmarking simulations of the Compton Spectrometer and Imager with calibrations. Nuclear Instruments and Methods in Physics Research A, volume 946, eid 192643, 28 pp.
Sleator, Clio C.; Zoglauer, Andreas; Lowell, Alexander W.; et al.
(See online at https://doi.org/10.1016/j.nima.2019.162643) - 2019: The Compton Spectrometer and Imager. Bulletin of the American Astronomical Society, volume 51, eid 98, 11 pp.
Tomsick, John; Zoglauer, Andreas; Sleator, Clio; et al.
- 2020: Detection of the 511 keV Galactic Positron Annihilation Line with COSI. The Astrophysical Journal, volume 895, issue 1, 44 pp.
Kierans, C. A.; Boggs, S. E.; Zoglauer, A.; Lowell, A. W.; et al.
(See online at https://doi.org/10.3847/1538-4357/ab89a9) - 2020: Imaging the 511 keV Positron Annihilation Sky with COSI. The Astrophysical Journal, volume 897, issue 1, 45 pp.
Siegert, Thomas; Boggs, Steven E.; Tomsick, John A.; et al.
(See online at https://doi.org/10.3847/1538-4357/ab9607) - 2021: COSI: From Calibrations and Observations to All-sky Images
Zoglauer, Andreas; Siegert, Thomas; Lowell, Alexander; et al.
- 2021: Study of 26Al in the COSI 2016 superpressure balloon flight. Proceedings of the 37th International Cosmic Ray Conference (ICRC 2021, Berlin)
Beechert, Jacqueline; Siegert, Thomas; Zoglauer, Andreas; et al.
(See online at https://doi.org/10.22323/1.395.0611)
