Counting the High-Energy Cosmic-Ray Electron Spectrum

The cosmic-ray electron spectrum is expanded to previously unheard-of high energies by a recent analysis of observations spanning more than ten years.

Despite being much less numerous than cosmic-ray protons and other nuclei, cosmic-ray electrons and positrons are crucial for understanding the high-energy processes occurring in our Galaxy. The system of high energy stereoscopic [H. E. A. A. By measuring the cosmic-ray electron and positron spectrum in great detail up to an astounding 40 TeV, Collaboration has made a major advancement in this field [1]. This spectrum was previously measured at energies lower than 5 TeV [2,]. The letter H. E. S. . S. . The team’s 12-year data set shows previously unheard-of detail, particularly around a noticeable “break” where the spectral slope steepens at about 1 TeV. Our knowledge of the origins of Galactic cosmic-rays is challenged by the findings, which verify that this break is one of the most noticeable and mysterious features throughout the entire cosmic-ray spectrum. Furthermore, there are substantial limitations on the contribution of local sources to the measured flux as well as on alternative production mechanisms, such as the potential annihilation or decay of dark matter particles in the Milky Way, due to the nearly featureless power law detected beyond the break and sustained over an entire order of magnitude in energy. We can now better understand local cosmic-ray accelerators and the propagation of high-energy particles throughout the Galaxy thanks to these ground-breaking measurements.

High-energy particles called cosmic-ray electrons rapidly lose energy as they move through the Galaxy, mostly due to interactions with background radiation and Galactic magnetic fields. Their range of propagation is limited by this energy loss, particularly at high energies, which increases the possibility of finding particle spectrum signatures from nearby cosmic-ray accelerators. We may discover the origins of cosmic rays by locating these nearby accelerators, which are most likely pulsars and supernova remnants. Exploring the high-energy end of the spectrum is also motivated by the possibility of finding cosmic-ray electrons created by unusual processes, like dark matter annihilation, which might be easier to see at higher energies where traditional astrophysical fluxes decrease.

The H is one of the main challenges in measuring cosmic-ray electrons, particularly when using indirect methods. A. A. S. . They are distinguished from the far more prevalent cosmic-ray protons and other nuclei by collaboration. The letter H. D. S. . S. Collaboration’s efforts are noteworthy due to the significant amount of high-quality data they gathered. utilizing a Namibian Cherenkov telescope array (Fig. To isolate electron events with high confidence, the team improved particle discrimination techniques to reach a proton rejection ratio of 10,000 to 1 (the spectrum also includes the contribution from cosmic-ray positrons). The resulting cosmic-ray electron spectrum is best described by a broken power law: the spectral index, or exponent of the power law, is roughly 3.25 below 1 TeV and sharply steepens to about 4.49 above 1 TeV (Fig. 2. . The cosmic-ray flux decreases faster at higher energies when the spectral index is higher.

Space-based experiments like CALET and DAMPE had previously identified the spectral break at 1 TeV [2, 3], but they were unable to extend measurements into the multi-TeV range, which is crucial for determining the origin of the break. At first, it was believed that energy losses as electrons moved through the Galaxy were the cause of this spectrum steepening. The boron-to-carbon ratio measured by AMS-02 (another space-based experiment), CALET, and DAMPE [4–6] is one example of a recent measurement of cosmic-ray nuclei that indicates the residence time of cosmic rays at this energy is incompatible with the break being primarily caused by simple energy loss. Moreover, H. E. S. . S. has demonstrated that the 1 TeV break is more abrupt than anticipated, which contradicts an origin based on the diffusive propagation of these particles through the Galaxy.

Another theory suggests that the break might be caused by the combined effect of a few local sources. However, measurements up to 40 TeV, H, reveal a featureless power law. D. S. . S. . restricts the role of these local sources, which should contribute bumps and valleys to the observed flux. Since there is currently no compelling explanation, these results are likely to prompt a reassessment of cosmic-ray acceleration models, particularly for electrons, in an effort to comprehend how Galactic accelerators energize these particles to relativistic speeds from the cold interstellar medium. This power law’s featurelessness beyond 1 TeV is particularly noteworthy because it lacks a clear peak at 1–4 TeV. Hints of this peak had previously been observed in DAMPE data, which some had hypothesized might point to a dark matter signature [7].

These findings have important ramifications. They reduce the number of possible candidates for cosmic-ray electron sources in the vicinity, for starters. Although dark matter annihilation is less likely to be the cause, more traditional explanations like pulsars or supernova remnants are still tenable. Moreover, this work poses fascinating queries concerning the laws regulating particle propagation at such high energies. Future studies will probably focus on improving particle discrimination even more, possibly using machine learning techniques, and expanding the direct measurements’ energy range to include even higher-energy electrons. The capital H. A. A. S. . There is still much to learn about the high-energy Universe, despite the fact that collaboration has raised the bar for cosmic-ray physics.

Citations.

F. Aharonian and associates. H. D. A. S. . cooperation), “High-statistics cosmic-ray electron spectrum measurement using H. D. A. S. . “Phys”. Rev. Allow. 133, 221001 (2024).

Oh. Adriani and associates. Calorimetric Electron Telescope on the International Space Station: Extended measurement of the cosmic-ray electron and positron spectrum from 11 GeV to 4.8 TeV (CALET Collaboration), Phys. Rev. Let’s. 120 (2018), 261102.

In Nature 552, 63 (2017), the DAMPE Collaboration reported the direct detection of a break in the teraelectronvolt cosmic-ray spectrum of electrons and positrons.

M.. Aguilar and colleagues. The Alpha Magnetic Spectrometer on the International Space Station was used to measure the boron to carbon flux ratio in cosmic rays with precision from 1point 9 GV to 2point 6 TV (AMS Collaboration), Phys. Rev. Let us. 117, 231102 (2016).

Okay. Adriani and others. (CALET Collaboration), “International Space Station Calorimetric Electron Telescope Measures Cosmic-Ray Boron Flux from 8.4 GeV/n to 3.8 TeV/n,” Phys. Rev. . Allow. 2022, 129, 2511003.

DAMPE Collaboration, “Using DAMPE to detect spectral hardenings in cosmic-ray boron-to-carbon and boron-to-oxygen flux ratios,” Sci. Bull. . 2162, 67 (2022).

Yep. -Z. Fan and others. Phys., “A model explaining neutrino masses and the DAMPE cosmic ray electron excess.”. Let us. (2018) B 781, 83.

About the Writer.

Theoretical astroparticle physicist Carmelo Evoli works in this field. He is an active member of the Pierre Auger Collaboration and an associate professor at the Gran Sasso Science Institute in Italy. In addition to his extensive studies in gamma-ray and neutrino astrophysics, his research focuses on cosmic rays with the goal of using a multimessenger approach to advance our understanding of Galactic and extragalactic cosmic-ray sources.

Subject Domains.