Discovered in 1912, cosmic rays have been widely studied and our current understanding of them is compiled in what is known as the Standard Model. Recently, this understanding has been challenged by the detection of unexpected spectral structures in the energy spectrum of cosmic ray protons. Now scientists are going further with high, low-uncertainty statistical measurements of these protons over a wider energy range using the Calorimetric Electron Telescope, confirming the presence of such structures.
Cosmic rays constitute high-energy protons and atomic nuclei that originate from stars (both in our galaxy and other galaxies) and are accelerated by supernovae and other high-energy astrophysical objects. Our current understanding of the energy spectrum of galactic cosmic rays suggests that it follows a power law dependency, in that the spectral index of protons detected in a certain energy range decreases according to the power law as as the energy increases. But recent observations made using magnetic spectrometers for low energies and calorimeters for high energies have hinted at a deviation from this power law variation, with the spectral index of protons becoming larger around an energy from a few hundred GeV to energies up to 10 TeV. . Following this “spectral hardening”, characterized by a lower absolute value of the spectral index, a “spectral softening” was detected above 10 TeV using the CALorimetric Electron Telescope (CALET), a space telescope installed at the International Space Station. However, better measurements with high statistics and low uncertainty should be performed over a broad energy spectrum for confirmation of these spectral structures.
That’s exactly what a team of international researchers led by Associate Professor Kazuyoshi Kobayashi of Waseda University in Japan set out to do. “With the data collected by CALET over approximately 6.2 years, we have put forward a detailed spectral structure of cosmic ray protons. The novelty of our data lies in the high statistical measurement over a wider energy range from 50 GeV to 60 TeV,” says Kobayashi. The results of their study, which included contributions from Professor Emeritus Shoji Torii of Waseda University (PI, or principal investigator, of the CALET project) and Professor Pier Simone Marrocchesi of the University of Siena in Italy, were published in the journal Physical Review Letters on September 1, 2022.
The new observations confirmed the presence of spectral hardening and softening below and above 10 TeV, suggesting that the proton energy spectrum is not consistent with a single variation of the law. of power for the whole range. Moreover, the spectral softening beginning at around 10 TeV is consistent with a previous measurement reported by the Dark Matter Particle Explorer (DAMPE) space telescope. Interestingly, the transition by spectral softening was found to be sharper than that by spectral hardening.
The variations and uncertainty of the new CALET data were checked using Monte Carlo simulations. The statistics were improved by a factor of approximately 2.2 and the spectral hardening characteristic was confirmed with a higher significance of more than 20 sigma.
Commenting on the importance of this research, Kobayashi remarked, “This result will significantly contribute to our understanding of the acceleration of cosmic rays by supernovae and the mechanism of cosmic ray propagation. The next step would be to extend our measurement of the proton spectrum to even higher levels. energies with reduced systematic uncertainties. This should be accompanied by a change in theoretical understanding to take account of new observations. »
Finally, it is not only about cosmic rays. On the contrary, the study continues to show how little we still understand about our Universe and that it is worth thinking about.
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Materials provided by Waseda University. Note: Content may be edited for style and length.