Tracking the most energetic electrons in cosmic rays

Cosmic radiation, discovered over a century ago, consists of a stream of particles (atomic nuclei, protons, electrons, photons, etc.) that constantly reach the Earth's atmosphere. These particles come from a multitude of astrophysical sources.

Since 2003, the H.E.S.S. observatory, built in Namibia by a consortium of 14 countries, has been operating 4 telescopes (and a fifth since 2012) to observe this cosmic radiation indirectly. When these high-energy particles hit the upper atmosphere, they interact with oxygen or nitrogen atoms, creating a cascade of other fast-moving particles. These generate a flash of light, undetectable to the naked eye, but captured by telescopes equipped with specially designed cameras, notably at the Laboratoire Leprince-Ringuet (LLR*).

In their latest publication, the scientists have analyzed in detail over a ten-year archive of observations, looking in particular for cosmic electrons (as well as positrons, which are far less common than electrons). Unlike photons, which travel through space in a straight line and whose origin can be identified, electrons are deflected by galactic magnetic fields, blurring their origin. “Moreover, cosmic electrons are all the rarer the higher their energy, making them difficult to detect,” points out Mathieu de Naurois, CNRS research director at LLR and deputy director of the H.E.S.S. collaboration.

By thoroughly sorting through several thousand hours of data, the researchers were able to extract these electrons, up to 40 Teraelectron volts (TeV). These are the most energetic cosmic electrons ever detected. By comparison, the record for electrons in particle accelerators on Earth is a hundred times lower.

Above all, analysis of the energy distribution of these electrons indicates that there is a break around 1 TeV, at which point the frequency of the observed electrons falls off more rapidly as energy increases. “This information, combined with the fact that electrons lose energy as they travel, tells us that the sources that emitted these particles are small in number and are located a few hundred light-years from the Solar System,” explains Mathieu de Naurois. Among the most likely hypotheses are pulsars (fast-rotating neutron stars) or supernova remnants. These extreme environments are known to be conducive to particle acceleration, as high-energy photons from such sources have already been detected. “But there may be energetic electrons at the origin of these photons, such as those highlighted by this study,” continues Mathieu de Naurois. It's yet another step towards understanding the mechanisms behind these phenomenal cosmic accelerations, many aspects of which are still unknown.

*LLR: a joint research unit CNRS, École Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France