Scientists have recently discovered electrons and positrons with the highest energy levels ever observed on our planet. This finding offers insights into cosmic phenomena that release enormous amounts of energy, though the exact sources of this energy remain uncertain.
The Universe is filled with extreme conditions, ranging from extremely cold temperatures to incredibly high energy environments. These conditions enable extraordinary entities, like supernova remnants, pulsars, and active galactic nuclei, to emit charged particles and gamma rays with energies that surpass those produced by nuclear fusion in stars by a significant margin.
The gamma rays we detect on Earth provide valuable information about these sources, as they can travel through space without interruption. However, charged particles, commonly referred to as cosmic rays, present a more complex situation. These particles are constantly affected by magnetic fields that exist throughout the Universe and arrive on Earth from all directions. Additionally, as they traverse space, they lose some energy when interacting with light and magnetic fields. This energy loss is particularly notable for the most energetic electrons and positrons, known as cosmic-ray electrons (CRe), which can exceed one teraelectronvolt (TeV)—an energy level that is 1,000 billion times greater than that of visible light.
Due to their nature, it is challenging to trace the origins of these charged particles, but their detection on Earth suggests there are powerful cosmic-ray particle accelerators nearby. However, identifying electrons and positrons with energies in the several teraelectronvolt range is quite difficult. Space-based detection tools, with areas only around one square meter, struggle to gather enough of these particles, as they become rarer with increasing energy. Conversely, ground-based instruments that detect cosmic rays by observing the particle showers they create in Earth’s atmosphere face the challenge of distinguishing between showers caused by cosmic-ray electrons (or positrons) and the more frequent showers created by heavier cosmic-ray protons and nuclei. The H.E.S.S. Observatory, situated in Namibia, employs five large telescopes to detect and record the faint Cherenkov radiation produced by charged particles and photons entering the Earth’s atmosphere, creating a shower of particles in their wake. While the Observatory primarily aims to detect gamma rays for research on their origins, the collected data can also be utilized to identify cosmic-ray electrons.
In an extensive analysis, scientists from the H.E.S.S. collaboration have gleaned new insights into the origin of these particles. The astrophysicists meticulously analyzed a vast dataset accumulated over ten years from the four 12-meter telescopes, applying advanced selection algorithms that enable them to efficiently extract cosmic-ray electrons from background noise. This groundbreaking approach generated an unparalleled statistical dataset for examining cosmic-ray electrons. Notably, the researchers successfully acquired data on cosmic-ray electrons even in the highest energy ranges, up to 40 TeV. This breakthrough allowed them to observe a distinct shift in the energy distribution of these cosmic-ray electrons.
“This discovery is significant because it suggests that the measured cosmic-ray electrons likely come from only a few sources located no more than a few thousand light years away from our solar system, a relatively small distance compared to the scale of our Galaxy,” states Kathrin Egberts from the University of Potsdam, a lead author of the study.
“This thorough analysis has allowed us to set strict limits on the origins of these cosmic electrons for the first time,” adds Prof. Hofmann from the Max-Planck Institute for Nuclear Physics and co-author of the study. “The extremely low fluxes at higher TeV energies hinder space missions from competing with this measurement. Hence, our findings not only provide critical data in an essential and previously unexplored energy range that enhances our understanding of the local cosmic environment but are also likely to serve as a reference point for years to come,” adds Mathieu de Naurois, a researcher at CNRS from the Laboratoire Leprince-Ringuet.
Footnotes :
- 1 TeV equals 1012 electronvolts.
- High-energy gamma rays can only be observed from the ground due to a unique phenomenon. When a gamma ray hits the atmosphere, it collides with atoms and molecules, generating new particles that cascade toward the ground like an avalanche. These particles emit brief flashes lasting billionths of a second (Cherenkov radiation), which can be detected using large, specialized telescopes on the ground. The H.E.S.S. Observatory, located in the Khomas Highlands of Namibia at an elevation of 1835 m, began operation in 2002 and comprises five telescopes. Four telescopes with 12-meter diameter mirrors are positioned at the corners of a square, with a larger 28-meter telescope at the center. This configuration enables the detection of cosmic gamma rays ranging from several tens of gigaelectronvolts (GeV, 109 electronvolts) to several tens of teraelectronvolts (TeV, 1012 electronvolts). For context, photons of visible light carry energy levels of two to three electronvolts. H.E.S.S. is currently the sole instrument monitoring high-energy gamma-ray light in the southern sky and is also the largest and most sensitive telescope system of its kind.
