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

Abstract

A precise measurement of the cosmic-ray proton spectrum with the Calorimetric Electron Telescope (CALET) is presented in the energy interval from 50 GeV to 60 TeV and the observation of a softening of the spectrum above 10 TeV is reported. The analysis is based on the data collected during ∼6.2 years of smooth operations aboard the International Space Station (ISS) and covers a broader energy range with respect to the previous proton flux measurement by CALET, with an increase of the available statistics by a factor of ∼2.2. Above a few hundred GeV we confirm our previous observation of a progressive spectral hardening with a higher significance (more than 20 sigma). In the multi-TeV region we observe a second spectral feature with a softening around 10 TeV and a spectral index change from −2.6 to −2.9 consistently, within the errors, with the shape of the spectrum reported by DAMPE. We apply a simultaneous fit of the proton differential spectrum which well reproduces the gradual change of the spectral index encompassing the lower energy power-law regime and the two spectral features observed at higher energies.

Formation of the Cosmic-Ray Halo: The Role of Nonlinear Landau Damping

“…Note added in proof. New data on CR proton spectrum reported by CALET (Adriani et al 2022) confirms the existence of the second spectral break at 10 TeV. The break position suggests that the scale height of hot gas should be about 2 kpc or less.…”

Direct Measurements of Galactic Cosmic Rays

“…More recently, another interesting and unexpected feature was observed from calorimeters DAMPE, CALET, NUCLEON and ISS-CREAM. It consists in the observation a new "knee", i.e., a spectral steepening, in the flux of CRs at about 10 TeV of energy [23][24][25][26][27]. Such a features has been observed in both protons and helium fluxes.…”

The High Energy Cosmic-Radiation Detection (HERD) facility for direct cosmic-ray measurements.

“…Recent measurements of the proton and helium spectra (the most abundant species in cosmic rays) have uncovered several unexpected departures from the single power law behavior predicted by conventional acceleration and propagation mechanisms. A hardening in both spectra at around 200-400 GeV/n [1, 2] followed by a softening in the multi-TeV region [3][4][5] are hints for significant physical processes contributing to acceleration and/or propagation in addition to standard ones. Similar spectral features have been discovered also in heavier nuclear species, with different behaviors between primary and secondary species [6] that suggest possible new features in propagation across the galaxy.…”