Scientists from the H.E.S.S. collaboration including a consortium of German universities, the Max-Planck-Institut für Kernphysik and the CNRS in France have recently identified electrons and positrons with the highest energies ever recorded on Earth. They provide evidence of cosmic processes emitting colossal amounts of energy, the origins of which are as yet unknown. These findings are due to be published on November 25 in the journal Physical Review Letters.
The universe is full of extreme environments, from the coldest temperatures to sources of the utmost energies possible. Extreme objects such as supernova remnants, pulsars or active galactic nuclei can produce charged particles and gamma-ray light, whose energies exceed those achievable in thermal processes such as nuclear fusion in stars by many orders of magnitude.
While the emitted gamma light travels undisturbed through space, the charged particles - or cosmic rays - are deflected by the omnipresent magnetic fields in the universe and reach the Earth isotropically from all directions. Furthermore, the charged particles loose parts of their energy along the way, interacting with light and magnetic fields. These losses are particularly severe for the most energetic electrons and positrons with energies above Tera-electronvolt (1 TeV = 1012 electronvolt), called cosmic-ray electrons (CRe). Their presence on Earth is therefore a clear indicator for the existence of powerful nearby cosmic particle accelerators, even if they cannot be used to trace their point of origin in space.
However, the detection of multi-Tera-electronvolt electrons and positrons is quite difficult: space instruments with a detection area of about one square metre cannot capture enough of the increasingly rare particles. Ground-based instruments can detect the particle cascades initiated when the cosmic particles hit the Earth’s atmosphere but face the challenge of identifying electron- or positron-initiated cascades among the much more frequent cascades generated by impacting heavier cosmic-ray nuclei. In 2008, researchers had first succeeded in identifying CRe’s in the data collected by the ground-based H.E.S.S. Cherenkov telescopes.
The H.E.S.S. observatory located in Namibia uses five large imaging atmospheric Cherenkov telescopes to record the faint Cherenkov light produced by highly charged particles and photons perpetrating our planet’s atmosphere and producing a particle cascade in their wake (fig. 1). While the primary aim of the H.E.S.S. observatory is to detect and select gamma rays, and to image their sources, the data can also be exploited to search for cosmic-ray electrons.
A new analysis presented by scientists from the H.E.S.S. collaboration was now able to obtain new insights into the origin of these particles. In their work the astrophysicists reassessed the huge data set collected during a decade by four of the H.E.S.S. telescopes and applied novel and strongent selection algorithms to identify CRe’s with an unprecedented low background contamination. This resulted in an unequalled high statistics data set for the analysis of the cosmic electrons. In particular, the researchers from the collaboration were able to obtain CRe data in the highest energy regimes up to 40 TeV (Tera-electronvolt) for the first time ever.
“We could observe that the energy spectrum of the CRe exhibits a smooth decline with increasing energy, the spectrum steepening markedly at approximately at 1 Tera-electronvolt. Both above and below this break, the spectrum follows a power law in energy, without exhibiting any additional features, as were predicted by many models for CRe acceleration”, remarks Mathieu de Naurois from the Laboratoire Leprince-Ringuet, E´cole Polytechnique, CNRS, one of the lead authors of the study.
The researchers found, however, that the transition from a shallow to a steep decline at about 1 Tera-electronvolt is surprisingly sharp.
“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. Emissions originating from many sources at different distances would wash out this signal considerably”, explains Kathrin Egberts from the University of Potsdam, co-author of the study. “We were able to put severe constrains on the origin of these cosmic electrons with our detailed analysis for the first time”.
Prof. Werner Hofmann from the Max-Planck-Institut für Kernphysik in Heidelberg explains the implications of the new analysis for astrophysical research: “The very low fluxes at higher energies place severe constraints on 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 neighbourhood, but it is also likely to remain a benchmark for the coming years”, he concludes.
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The H.E.S.S. Observatory
High-energy gamma rays can only be observed from the ground with a trick. When a gamma ray enters the atmosphere, it collides with atoms and molecules and generates new particles that race on towards the ground like an avalanche. These particles emit flashes lasting only a few billionths of a second (Cherenkov radiation), which can be observed with specially equipped large telescopes on the ground. High-energy gamma astronomy therefore uses the atmosphere like a giant fluorescent screen. The H.E.S.S. observatory, located in the Khomas Highlands of Namibia at an altitude of 1835m, officially went into operation in 2002. It consists of an array of five telescopes. Four telescopes with mirror diameters of 12 m are located at the corners of a square, with a further 28 m telescope in the center. This makes it possible to detect cosmic gamma radiation in the range of a few tens of Giga-electronvolts (GeV, 109 electronvolts) to a few tens of Tera-electronvolts (TeV, 1012 electronvolts). For comparison: visible light particles have energies of two to three electron volts. H.E.S.S. is currently the only instrument that observes the southern sky in high-energy gamma light and is also the largest and most sensitive telescope system of its kind.
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Bibliography :
High-Statistics Measurement of the Cosmic-Ray Electron Spectrum with H.E.S.S.
H.E.S.S. Collaboration. Physical Review Letters, November 25, 2024.
DOI: 10.1103/PhysRevLett.133.221001
arXiv: http://arxiv.org/abs/2411.08189
Link to the international press release:
