Cosmic rays were discovered in 1912 and have been widely studied, and our current understanding of them is gathered 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 the researchers are moving forward 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 are 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 dependence, with the spectral index of protons detected in a particular energy range decreasing according to the power law as the energy increases. But recent observations made using magnetic spectrometers for low energies and calorimeters for high energies have suggested a departure from this power-law variation, where the spectral index of protons becomes larger around an energy from a few hundred GeV to energies up to 10 TeV. . After 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 on the International Space Station. However, better measurements with high statistics and low uncertainty should be performed over a wide energy spectrum to confirm these spectral structures.
This is exactly what a team of international researchers led by associate professor Kazuyoshi Kobayashi from Waseda University in Japan set out to do. “With data collected by CALET over approximately 6.2 years, we have presented 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, was 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 does not conform to a single variation of the power law for the entire range. Furthermore, the spectral softening that begins at about 10 TeV is consistent with a previous measurement reported by the Dark Matter Particle Explorer (DAMPE) space telescope. Interestingly, the transition in spectral softening was found to be sharper than in spectral hardening.
The variations and uncertainties in the new CALET data were checked using Monte Carlo simulations. The statistics were improved by a factor of approx. 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 noted: “This result will contribute significantly to our understanding of the acceleration of cosmic rays from 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 account for new observations.”
Finally, it’s not just 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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