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A pulsar within a few thousand light-years of Earth could have accelerated electrons and positrons to the extreme energies now measured by the H.E.S.S.-Observatory

Five telescopes of the H.E.S.S.-collaboration in Namibia are used to study cosmic radiation, especially gamma radiation. In data from ten years of observations, the researchers have now been able to detect cosmic electrons and positrons with an unprecedented energy of more than ten tera-electronvolts (1 TeV corresponds to 10^12 electronvolts). Since charged particles are deflected in all directions by the magnetic fields in our cosmic neighborhood, it is difficult to determine their origin. This time, however, the outstanding quality of the measured particle energy spectrum up to the highest energy values opens up new possibilities: the scientists suspect that a pulsar, which may be no more than a few thousand light-years away, could be the source.

The Universe hosts extreme environments, from the coldest temperatures to the most energetic sources. Extreme objects such as supernova remnants, pulsars or active galactic nuclei produce charged particles and gamma radiation with energies far above those reached in thermal processes such as nuclear fusion in stars.

While the emitted gamma-rays cross space undirturbed, the charged particles – or cosmic rays – are deflected by the omnipresent magnetic fields in the universe and reach the Earth isotropically from all directions. This means that researchers cannot directly deduce the origin of the radiation. In addition, charged particles lose energy through interactions with light and magnetic fields. These losses are particularly strong for the most energetic electrons and positrons (positively charged anti-particles of the electron) with energies above the tera-electronvolt mark. When instruments on Earth measure charged cosmic particles of such high energies, it means that they cannot have traveled far. This points to the existence of powerful natural particle accelerators near our solar system.

A kink in the spectrum reveals the origin

In a new analysis, scientists from the H.E.S.S. collaboration have for the first time narrowed down where these cosmic particles come from. The starting point of the analysis is the measurement of the spectrum of cosmic rays, i.e. the energy distribution of the measured electrons and positrons. The analysis is based on ten years of observations, which guarantees high data quality. The integrated electron spectrum extends up to several tens of tera-electronvolts. “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”, says Werner Hofmann of the Max Planck Institute for Nuclear Physics in Heidelberg. In the spectrum, which is characterized by comparatively small error bars at TeV energies, a prominent kink at around one tera-electronvolt is striking. Both above and below this break, the spectrum follows a power law without any further anomalies.

Straying through the galaxy

To find out which astrophysical process has accelerated the electrons to such high energies and what the origin of the kink is, the researchers compare these data with model predictions. Source candidates are pulsars, which are stellar remnants with strong magnetic fields. Some pulsars blow a wind of charged particles into their surroundings, and the magnetic shock front of this wind could be the place where the particles experience a boost. The same applies to shock fronts of supernova remnants. Computer models show that electrons accelerated in this way travel into space with a certain energy distribution. These models track the electrons and positrons as they move through the Milky Way and calculate how their energy changes as they interact with magnetic fields and light in the Milky Way. In the process, the particles lose so much energy that their original energy spectrum is distorted. In the final step, astrophysicists try to fit their model to the data in order to learn more about the nature of the astrophysical sources.

But what object has hurled the electrons into space that the telescopes have measured? The particle spectrum with energies below one tera-electronvolt probably consists of electrons and positrons from different pulsars or supernova remnants. At higher energies, however, a different picture emerges: the energy spectrum drops steeply from about one teraelectronvolt. This is also confirmed by models that study the particles accelerated by astronomical sources and their diffusion by the galactic magnetic field. This transition at one tera-electronvolt is particularly pronounced and exceptionally sharp. “This is an important result, as we can conclude that the measured electrons 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”, says Kathrin Egberts of the University of Potsdam. This distance is relatively small compared to the size of the Milky Way. “Sources at different distances would wash out this kink considerably”, Egberts continues. According to Werner Hofmann, even a single pulsar could be responsible for the electron spectrum at high energies. However, it is not clear which one that is. Since the source must be very close by, only a few pulsars come into question.

RH/BEU

Background Information

Data analysis: The astrophysicists analyzed a huge data set collected over a decade by four of the H.E.S.S. telescopes. They used novel and rigorous selection algorithms to identify cosmic electrons with unprecedentedly low background contamination. This resulted in a statistically high-quality dataset for the analysis of cosmic electrons. In particular, the researchers were able to measure electrons and positrons with energies of up to 40 TeV.

Detection method: Detecting high-energy, charged cosmic particles is difficult. Space-based telescopes with a detector area of about one square meter do not capture enough of the rare particles. Ground-based instruments use a trick: when a gamma ray or a fast, charged particle enters the atmosphere, it collides with atoms and molecules, creating new particles that rush to Earth like an avalanche. In this particle cascade, individual particles produce flashes of light (Cherenkov radiation), which can be observed with specialized large telescopes on the ground. High-energy astronomy thus uses the atmosphere as a giant detector.

The challenge is to distinguish the cascades produced by electrons or positrons from the more common cascades produced by the impact of heavier cosmic nuclei or gamma photons. In 2008, researchers succeeded for the first time in identifying electron and positron signals in the data from the H.E.S.S.-Cherenkov-Telescope.

Source identification: While gamma radiation can be traced directly back to the source, this is not possible for charged cosmic particles. These hit the Earth’s atmosphere from a wide range of directions, even if they all originate from the same source. This is due to the deflection by magnetic fields in the Milky Way.

The H.E.S.S.-Observatory: The H.E.S.S.-Observatory in the Khomas Highlands of Namibia, at an altitude of 1,835 meters, began operations in 2002. It consists of an array of five telescopes: four 12-meter telescopes at the corners of a square and a further 28-meter telescope in the center. This allows cosmic gamma radiation in the range from a few tens of gigaelectronvolts (GeV, 10^9 electronvolts) to a few tens of teraelectronvolts (TeV, 10^12 electronvolts) to be detected. For comparison: particles of visible light have an energy of two to three electronvolts. 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.

Although the H.E.S.S.-Observatory is primarily used to detect and select gamma radiation and to measure its sources, the data obtained can also be used to search for cosmic electrons.