Direct Measurement of the Spectral Structure of Cosmic-Ray $\text{Electrons}+\text{Positrons}$ in the TeV Region with CALET on the International Space Station

Detailed measurements of the spectral structure of cosmic-ray electrons and positrons from 10.6 GeV to 7.5 TeV are presented from over 7 years of observations with the CALorimetric Electron Telescope (CALET) on the International Space Station. The instrument, consisting of a charge detector, an imaging calorimeter, and a total absorption calorimeter with a total depth of 30 radiation lengths at normal incidence and a fine shower imaging capability, is optimized to measure the all-electron spectrum well into the TeV region. Because of the excellent energy resolution (a few percent above 10 GeV) and the outstanding $\mathrm{e}/\mathrm{p}$ separation (${10}^5$), CALET provides optimal performance for a detailed search of structures in the energy spectrum. The analysis uses data up to the end of 2022, and the statistics of observed electron candidates has increased more than 3 times since the last publication in 2018. By adopting an updated boosted decision tree analysis, a sufficient proton rejection power up to 7.5 TeV is achieved, with a residual proton contamination less than 10%. The observed energy spectrum becomes gradually harder in the lower energy region from around 30 GeV, consistently with AMS-02, but from 300 to 600 GeV it is considerably softer than the spectra measured by DAMPE and Fermi-LAT. At high energies, the spectrum presents a sharp break around 1 TeV, with a spectral index change from $-3.15$ to $-3.91$, and a broken power law fitting the data in the energy range from 30 GeV to 4.8 TeV better than a single power law with 6.9 sigma significance, which is compatible with the DAMPE results. The break is consistent with the expected effects of radiation loss during the propagation from distant sources (except the highest energy bin). We have fitted the spectrum with a model consistent with the positron flux measured by AMS-02 below 1 TeV and interpreted the $\mathrm{electron}+\text{positron}$ spectrum with possible contributions from pulsars and nearby sources. Above 4.8 TeV, a possible contribution from known nearby supernova remnants, including Vela, is addressed by an event-by-event analysis providing a higher proton-rejection power than a purely statistical analysis.

Figure 1

An example of BDT response distributions in the $754<E<949\mathrm{GeV}$ bin, including all acceptance conditions.

Figure 2

Cosmic-ray all-electron spectrum measured by CALET from 10.6 GeV to 7.5 TeV using the same energy binning as in our previous publication below 4.8 TeV [15], where the gray band indicates the quadratic sum of statistical and systematic errors (not including the uncertainty on the energy scale). Also plotted are other direct measurements in space [16, 29, 30] for comparison. The enlarged figure is shown in Fig. S4 in the Supplemental Material [20].

Figure 3

All-electron spectrum measured by CALET from 10.6 GeV to 7.5 TeV, and the fitted results in the energy range from 30 GeV to 4.8 TeV, with a broken power law, an exponentially cutoff power law and a single power law. The error bars represent statistical and systematic uncertainties except normalization. See text for the details of the fits by power laws.

Figure 4

Possible spectral fit over the whole region of CALET observations, including pulsars and nearby SNR sources as individual sources, with the Vela SNR dominating in the TeV region. See details in text.