Energy Spectrum of Cosmic-Ray Electron and Positron from 10 GeV to 3 TeV Observed with the Calorimetric Electron Telescope on the International Space Station

First results of a cosmic-ray electron and positron spectrum from 10 GeV to 3 TeV is presented based upon observations with the CALET instrument on the International Space Station starting in October, 2015. Nearly a half million electron and positron events are included in the analysis. CALET is an all-calorimetric instrument with total vertical thickness of 30 $X_0$ and a fine imaging capability designed to achieve a large proton rejection and excellent energy resolution well into the TeV energy region. The observed energy spectrum over 30 GeV can be fit with a single power law with a spectral index of $-3.152\pm 0.016$ ($\text{stat}+\text{syst}$). Possible structure observed above 100 GeV requires further investigation with increased statistics and refined data analysis.

Figure 1

A schematic side view of the main calorimeter. An example of a simulated 1 TeV electron event is superimposed to illustrate the shower development in the calorimeter.

Figure 2

An example of a 1 TeV electron shower candidate in flight data.

Figure 3

An example of BDT response distributions in the $476<E<599\mathrm{GeV}$ bin. The reduced chi square in the BDT response range from $-0.5$ to 0.5 is obtained as 1.14.

Figure 4

Stability of BDT analysis with respect to independent training samples and BDT-cut efficiency in the $949<E<1194\mathrm{GeV}$ bin. Color maps show the flux ratio dependence on efficiency, where the bin value (number of trials) increases as color changes from violet, blue, green, yellow, to red. A projection onto the $Y$ axis is shown as a rotated histogram (in gray color).

Figure 5

Energy dependence of systematic uncertainties. The red squares represent the systematic uncertainties stemming from the electron identification based on BDT. The bands defined by black lines show the sum in quadrature of all the sources of systematics, except the energy scale uncertainties.

Figure 6

Cosmic-ray all-electron spectrum measured by CALET from 10 GeV to 3 TeV, where systematic errors (not including the uncertainty on the energy scale) are drawn as a gray band. The measured all-electron flux including statistical and systematic errors is tabulated in Ref. [6]. Also plotted are measurements in space [35, 36, 37] and from ground-based experiments [38, 39].