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

TL;DR

This paper presents a precise measurement of cosmic-ray proton spectrum from 50 GeV to 60 TeV using CALET on the ISS, revealing spectral features including a softening above 10 TeV and a hardening below, with results consistent with DAMPE.

Contribution

It provides the most extensive and precise measurement of the proton spectrum up to 60 TeV, confirming spectral features with higher significance and comparing favorably with DAMPE observations.

Findings

Spectral softening observed above 10 TeV

Progressive spectral hardening below a few hundred GeV

Spectral index change from 2.6 to 2.9 around 10 TeV

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 $\sim$6.2 years of smooth operations aboard the International Space Station 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 $\sim$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.