Gamma rays provide transparent information about cosmic sources as they travel through space without disturbance. However, cosmic rays, which are charged particles, are more complex.
They are affected by magnetic fields throughout the Universe and arrive at Earth from all directions. These particles also lose energy as they interact with light and magnetic fields, particularly the most energetic ones—cosmic-ray electrons (CRe) with energies exceeding one teraelectronvolt (TeV).
Because of these energy losses, it’s impossible to pinpoint their exact origin in space. Nevertheless, their detection on Earth suggests the presence of powerful cosmic-ray accelerators nearby.
Detecting electrons and positrons with energies in the teraelectronvolt range is particularly difficult. Space-based instruments, with detection areas of around one square meter, cannot capture enough of these rare particles.
Ground-based instruments, which detect cosmic rays through the particle showers they produce in Earth’s atmosphere, face distinguishing between showers caused by cosmic-ray electrons (or positrons) and the more common showers from heavier cosmic-ray protons and nuclei. The H.E.S.S. Observatory in Namibia, equipped with five large telescopes, detects the faint Cherenkov radiation produced by charged particles and photons entering the atmosphere. Although primarily focused on gamma-ray detection, the data from H.E.S.S. can also be used to search for cosmic-ray electrons.
In the most extensive analysis ever conducted, scientists from the H.E.S.S. collaboration have gained new insights into the origin of cosmic-ray electrons. By analyzing a vast data set collected over a decade using four 12-meter telescopes, they applied advanced selection algorithms to isolate cosmic-ray electrons (CRe) from background noise efficiently.
This provided an unprecedented statistical dataset for studying cosmic-ray electrons. For the first time, the researchers obtained data on CRe up to the highest energy ranges, reaching 40 TeV. This analysis revealed a surprisingly sharp break in the energy distribution of these cosmic-ray electrons.
Kathrin Egberts, from the University of Potsdam, one of the study’s corresponding authors, said, “This is an important result, as we can conclude that the measured CRe most likely originates from very few sources in the vicinity of our own solar system, up to a maximum of a few thousand light years away, a very small distance compared to the size of our Galaxy.”
Prof. Hofmann, from the Max-Planck-Institut für Kernphysik, co-author of the study, said, “For the first time, our detailed analysis was able to put severe constraints on the origin of these cosmic electrons.”
Mathieu de Naurois, CNRS Researcher from the Laboratoire Leprince-Ringuet, adds, “The very low fluxes at larger TeV limit the possibilities of space-based missions to compete with this measurement. Therefore, our measurement not only provides data in a crucial and previously unexplored energy range, impacting our understanding of the local neighbourhood, but it is also likely to remain a benchmark for the coming years.”
Journal Reference:
- F. Aharonian, F. Ait Benkhali et al. High-Statistics Measurement of the Cosmic-Ray Electron Spectrum with H.E.S.S. Physical Review Letters. DOI: 10.1103/PhysRevLett.133.221001