The Earth is constantly being hit by high-energy particles from space. However, it is still unclear where this cosmic radiation comes from – this applies especially to its most energetic particles because they are rare and difficult to detect. Now, with the help of the HESS observatory in Namibia, astronomers have succeeded for the first time in detecting cosmic electrons with record energies of up to 40 teraelectron volts. This evidence provides new insights into the high-energy portion of cosmic rays. The frequency and energy distribution of the electrons and positrons flowing in from space also shows a strikingly sharp bend at around one teraelectron volt. This confirms previous measurements and could indicate a more local cosmic source, as the team explains.
Whether supernovae, the eruptions around active black holes or the concentrated outflows of pulsars: many events and objects in space release high-energy streams of radiation and charged particles. Our sun and other stars are also included. However, while the solar wind belongs to the lower-energy portion of this cosmic radiation and has been relatively well studied, this does not apply to the most energetic charged particles of cosmic radiation. There are two reasons for this: Firstly, cosmic electrons and other particles become rarer the higher their energy is. Astronomers therefore need the largest possible detectors to obtain information about the density and origin of these particles. On the other hand, the cosmic electrons in particular are strongly deflected and slowed down by interaction with the magnetic fields present everywhere in the universe. This makes it nearly impossible to locate their sources.
The problem of detectors
“The locally arriving electrons and positrons of cosmic rays could either originate from one or more primary sources in the neighborhood,” explain Felix Aharonian from the Max Planck Institute for Nuclear Physics in Heidelberg and his colleagues. However, so far it has not been possible to find clear evidence of such local sources in the spectrum of the detected cosmic electrons. “The high-energy electrons could therefore also be of secondary origin and arise from the interaction of cosmic ray atomic nuclei with interstellar gases,” the researchers continued. It is unclear which of the two assumptions is correct – also because there is currently a lack of data on the most energetic electron component of cosmic rays. Measuring instruments, including the Fermi LAT space telescope and the AMS-2 detector on the International Space Station, have so far only captured electrons with a maximum of a few teraelectron volts of energy.
Earth-based detectors, on the other hand, have the problem that they cannot measure cosmic radiation directly because the particles collide with atoms high up in the atmosphere and are intercepted. This creates cascades of secondary particles that can be detected by the measuring instruments. Finding out which type of cosmic particles originally generated these secondary showers – whether from electrons or heavier atomic nuclei or ions – therefore requires complex analysis. The HESS Observatory in Namibia is one of the earth-based observatories. It consists of four twelve-meter reflecting telescopes and a fifth, central 28-meter telescope. Their sensors are designed to detect the weak glow that occurs when the particle shower collides with air particles, the so-called Cherenkov radiation.
Electrons with up to 40 teraelectronvolts of energy
For their current study, Aharonian and his colleagues from the HESS collaboration have now specifically searched for high-energy cosmic electrons and positrons for twelve years of HESS measurements. They used special selection algorithms to filter these particles out of the background noise. They found what they were looking for: “Our analysis revealed 265,574 electron-like events in the energy range from 0.3 to 40 teraelectron volts,” report the researchers. They have thus detected the most energetic cosmic electrons and positrons to date. “The HESS data thus expand the spectrum of high-energy cosmic electron events far beyond previous direct and indirect measurements,” writes the team. For the first time, these new data provide an insight into how the frequency of these particles develops in the energy range beyond a few teraelectron volts.
As the analyzes showed, the curve follows a uniform trend across the entire energy sector – with one exception. At around one teraelectron volt, the energy distribution of the cosmic electrons shows a surprisingly sharp bend. Accordingly, significantly more electrons of this energy arrive on Earth than corresponds to the otherwise consistent trend. “This is an important result because we can conclude that the measured particles most likely come from only a few sources near our own solar system,” explains co-author Kathrin Egberts from the University of Potsdam. According to the calculations, the origins of the high-energy cosmic electrons are a maximum of a few thousand light-years away. “With our detailed analysis, we were able to significantly restrict the origin of these cosmic electrons for the first time,” adds Aharonian’s colleague Werner Hofmann. However, it is still not clear which events or objects release these electrons.
Source: Felix Aharonian (Max Planck Institute for Nuclear Physics, Heidelberg) et al., Physical Review Letters