At high energies, cosmic ray protons display novel spectrum structures

Observation of Spectral Structures in the Flux of Cosmic-Ray Protons from 50 GeV to 60 TeV with the Calorimetric Electron Telescope on the International Space Station. Credit: Waseda University

Cosmic rays are high-energy protons and atomic nuclei emitted by stars (both inside and beyond our galaxy) and accelerated by supernovae and other high-energy astronomical events. According to our present knowledge of the Galactic cosmic ray energy spectrum, it follows a power-law dependency, which means that the spectral index of protons observed within a specific energy range decreases by a power law as energy increases.

Recent measurements employing magnetic spectrometers for low energy levels and calorimeters for high energy levels, however, have suggested a divergence from this power-law pattern, with the spectral index of protons increasing greater around a few hundred GeV at energies up to 10 TeV. Following this spectral hardness, which is characterized by a lower absolute value of the spectral index, a spectral softening over 10 TeV has been seen using the CALorimetric Electron Telescope (CALET), a space telescope placed on the International Space Station. However, improved measurements with good statistics and low uncertainty over a broad energy spectrum are required to establish these spectral patterns. This is precisely what an international team of academics led by Associate Professor Kazuyoshi Kobayashi of Waseda University in Japan set out to do.

According to Kobayashi, we have presented a detailed spectral structure of the cosmic ray protons using data collected by CALET over approximately 6.2 years. The interesting aspect of our data is the high-statistics measurement over a larger energy range of 50 GeV to 60 TeV.

The findings of their study, which included contributions from Waseda University's Professor Emeritus Shoji Torii (PI, or Principal Investigator, of the CALET project) and the University of Siena in Italy's Professor Pier Simone Marrocchesi, was published in the journal Physical Review Letters. The new findings verified the occurrence of spectral hardening and softening below and above 10 TeV, implying that the proton energy spectrum does not follow a single power law variation throughout the whole range. Furthermore, the spectrum softening begins at about 10 TeV, which is consistent with a recent measurement made by the Dark Matter Particle Explorer (DAMPE) space telescope. Surprisingly, the transition caused by spectral softening was found to be sharper than the transition caused by spectral hardness.

Monte Carlo simulations were used to control the variability and uncertainty in the new CALET data. The statistics were enhanced by a factor of around 2.2, and the spectral hardening characteristic was verified with greater than 20 sigmas of significance. When asked about the relevance of this discovery, Kobayashi said that it will help us better understand cosmic ray acceleration by supernovae and cosmic ray propagation mechanisms. The next step would be to expand our measurement of proton spectra to higher energy while reducing systematic errors. This should be followed by a shift in theoretical understanding to account for the new findings. But it's not only about cosmic rays. Rather, the study goes on to highlight how much we still don't know about our cosmos, and why it's valuable to think about it.

Journal Information: O. Adriani et al, Observation of Spectral Structures in the Flux of Cosmic-Ray Protons from 50 GeV to 60 TeV with the Calorimetric Electron Telescope on the International Space Station, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.101102