Reviewed by Lexie CornerNov 26 2024
Researchers from the CNRS, a consortium of German universities, and the Max-Planck-Institut für Kernphysik, collaborating at the H.E.S.S. observatory, have detected electrons and positrons with the highest energies ever observed on Earth. This discovery provides insights into cosmic processes that release vast amounts of energy, the sources of which remain unknown. The findings are published in Physical Review Letters.
Visualization of the H.E.S.S. telescope array capturing the showers of particles produced by high-energy cosmic electrons and positrons, as well as gamma rays. Image Credit: ©Collaboration MPIK/H.E.S.S.
The Universe is filled with extreme environments, ranging from the coldest temperatures to the most intense energy sources imaginable. These conditions give rise to phenomena such as supernova remnants, pulsars, and active galactic nuclei, which can emit charged particles and gamma rays with energies far exceeding those generated by nuclear fusion in stars.
Gamma rays detected on Earth offer valuable insights into their sources, as they travel through space without interference. In contrast, charged particles, or cosmic rays, present a more complex scenario.
These particles are constantly deflected by magnetic fields that permeate the Universe, causing them to reach Earth isotropically—coming from all directions. Additionally, charged particles lose energy as they interact with light and magnetic fields during their journey. This energy loss is especially significant for the most energetic electrons and positrons, known as cosmic-ray electrons (CRe), with energies exceeding one teraelectronvolt (TeV), or 1,000 billion times that of visible light.
As a result, pinpointing the origin of these charged particles in space is not possible. However, their detection on Earth strongly suggests the presence of powerful cosmic-ray particle accelerators in the nearby vicinity.
Detecting electrons and positrons with energies reaching several teraelectronvolts presents significant challenges. Space-based instruments, which typically have detection areas of about 1 square meter, struggle to capture enough of these particles, as they become increasingly rare at higher energies.
Ground-based instruments, on the other hand, rely on detecting cosmic rays indirectly by observing the particle showers they generate in Earth's atmosphere. However, distinguishing showers caused by cosmic-ray electrons or positrons from the more common showers produced by heavier cosmic-ray protons and nuclei remains a challenge.
The H.E.S.S. Observatory, located in Namibia, uses five large telescopes to detect and record faint Cherenkov radiation produced by charged particles and photons entering Earth's atmosphere, which then generate particle showers. While the Observatory's primary goal is to detect and study gamma rays to investigate their origins, its data can also be used to search for cosmic-ray electrons.
In the most comprehensive analysis to date, scientists from the H.E.S.S. collaboration have uncovered new insights into the origins of cosmic-ray electrons. By examining extensive data collected over a decade by the four 12-meter telescopes, the team applied advanced selection algorithms to isolate CRe from background noise with unprecedented precision. This effort resulted in a unique dataset for studying CRe.
Notably, the researchers obtained data on CRe at the highest energy levels, reaching up to 40 TeV, for the first time. This breakthrough allowed them to identify an unexpectedly sharp break in the energy distribution of cosmic-ray electrons.
This is an important result, as we can conclude that the measured CRe most likely originate from very few sources in the vicinity of our own solar system, up to a maximum of a few 1000 light years away, a very small distance compared to the size of our Galaxy.
Kathrin Egberts, Study Co-Corresponding Author, University of Potsdam
“We were able to put severe constraints on the origin of these cosmic electrons with our detailed analysis for the first time,” notes Prof. Hofmann from the Max-Planck-Institut für Kernphysik, co-author of the study.
The very low fluxes at larger TeV limit the possibilities of space-based missions to compete with this measurement. Thereby, our measurement does not only provide data in a crucial and previously unexplored energy range, impacting our understanding of the local neighborhood, but it is also likely to remain a benchmark for the coming years.
Mathieu de Naurois, CNRS Researcher, Laboratoire Leprince-Ringuet
Journal Reference:
Aharonian, F., et al. (2024) High-Statistics Measurement of the Cosmic-Ray Electron Spectrum with H.E.S.S. Physical Review Letters. doi.org/10.1103/physrevlett.133.221001.