Towards Understanding the Origin of Cosmic-Ray Electrons

Precision results on cosmic-ray electrons are presented in the energy range from 0.5 GeV to 1.4 TeV based on $28.1\times {10}^6$ electrons collected by the Alpha Magnetic Spectrometer on the International Space Station. In the entire energy range the electron and positron spectra have distinctly different magnitudes and energy dependences. The electron flux exhibits a significant excess starting from $42.1_{-5.2}^{+5.4}\mathrm{GeV}$ compared to the lower energy trends, but the nature of this excess is different from the positron flux excess above $25.2\pm 1.8\mathrm{GeV}$. Contrary to the positron flux, which has an exponential energy cutoff of $810_{-180}^{+310}\mathrm{GeV}$, at the $5\sigma$ level the electron flux does not have an energy cutoff below 1.9 TeV. In the entire energy range the electron flux is well described by the sum of two power law components. The different behavior of the cosmic-ray electrons and positrons measured by the Alpha Magnetic Spectrometer is clear evidence that most high energy electrons originate from different sources than high energy positrons.

Figure 1

The AMS electron (blue data points) and positron (red data points, multiplied by 10) spectra (${\tilde{E}}^3{\mathrm{\Phi }}_{e^{\pm }}$). For display purposes the electron data point at $\sim 830\mathrm{GeV}$ is slightly shifted horizontally to avoid overlap with the positron point. As seen, the electron spectrum has distinctly different magnitude and energy dependence compared to that of positrons.

Figure 2

(a) The AMS electron spectrum (${\tilde{E}}^3{\mathrm{\Phi }}_{e^{-}}$, red data points, placed at $\tilde{E}$) and (b) the AMS positron fraction (red data points, placed at the bin center). Also shown are earlier measurements from PAMELA [29], Fermi-LAT [30], MASS [31], CAPRICE [32], AMS-01 [24, 33], and HEAT [34]. (c) The sum of AMS electron and positron [4] spectra (red data points, placed at $\tilde{E}$). Also shown are recent measurements of the combined $(e^{+}+e^{-})$ flux from ATIC [35], HESS [36], Fermi-LAT [37], DAMPE [38], and CALET [39].

Figure 3

(a) The spectral indices of the AMS electron (blue data points) and positron [4] (red data points) fluxes in nonoverlapping energy intervals. They show distinctly different behavior. The blue band represents the 68% C.L. interval of the fit of Eq. (5) to the electron flux (see text). The green band represents the 68% C.L. interval of a fit to the positron flux [Eq. (4) in Ref. 4]. (b) A double power law fit of Eq. (3) to the electron flux in the energy range [20.04–1400] GeV. The blue data points are the measured electron flux values scaled by ${\tilde{E}}^3$. The fitted function is represented by the solid red line. The vertical dashed line and the red band correspond to the value and the error of the energy $E_0$ where the change of the spectral index occurs and $\mathrm{\Delta }\gamma =0.094\pm 0.014$ is the magnitude of the spectral index change; see Eq. (3). The dashed red line is the extrapolation of the power law below $E_0$ into the higher energy region [or $\mathrm{\Delta }\gamma =0$ in Eq. (3)].

Figure 4

The two power law fit of Eq. (5) to the electron flux data in the energy range [0.5–1400] GeV with 68% C.L. (green band). The two power law components $a$ and $b$ of Eq. (5) are represented by the gray and blue areas, respectively. Also shown are the positron spectrum together with the fit of Eq. (4) of Ref. [4] including the positron diffuse (i.e., interstellar secondary production) term (the brown area) and positron source term (the magenta area) contributions.