Electron and Positron Fluxes in Primary Cosmic Rays Measured with the Alpha Magnetic Spectrometer on the International Space Station

Precision measurements by the Alpha Magnetic Spectrometer on the International Space Station of the primary cosmic-ray electron flux in the range 0.5 to 700 GeV and the positron flux in the range 0.5 to 500 GeV are presented. The electron flux and the positron flux each require a description beyond a single power-law spectrum. Both the electron flux and the positron flux change their behavior at $\sim 30\mathrm{GeV}$ but the fluxes are significantly different in their magnitude and energy dependence. Between 20 and 200 GeV the positron spectral index is significantly harder than the electron spectral index. The determination of the differing behavior of the spectral indices versus energy is a new observation and provides important information on the origins of cosmic-ray electrons and positrons.

Figure 1

The AMS (a) electron and (b) positron fluxes, multiplied by ${\tilde{E}}^3$. Statistical and systematic uncertainties of the AMS results have been added in quadrature. Also shown are the most recent measurements from PAMELA [9] and Fermi-LAT [10].

Figure 2

Detailed AMS (a) electron and (b) positron fluxes, multiplied by ${\tilde{E}}^3$, up to 200 GeV, with earlier measurements by PAMELA [9], Fermi-LAT [10], MASS [11], CAPRICE [12], AMS-01 [13], and HEAT [14].

Figure 3

The spectral indices of the electron flux ${\gamma }_{e^{-}}$ and of the positron flux ${\gamma }_{e^{+}}$ as a function of energy. The shaded regions indicate the 68% C.L. intervals including the correlation between neighboring points due to the sliding energy window.