Cosmic-Ray Electron Flux Measured by the PAMELA Experiment between 1 and 625 GeV

Precision measurements of the electron component in the cosmic radiation provide important information about the origin and propagation of cosmic rays in the Galaxy. Here we present new results regarding negatively charged electrons between 1 and 625 GeV performed by the satellite-borne experiment PAMELA. This is the first time that cosmic-ray $e^{-}$ have been identified above 50 GeV. The electron spectrum can be described with a single power-law energy dependence with spectral index $-3.18\pm 0.05$ above the energy region influenced by the solar wind ($>30\mathrm{GeV}$). No significant spectral features are observed and the data can be interpreted in terms of conventional diffusive propagation models. However, the data are also consistent with models including new cosmic-ray sources that could explain the rise in the positron fraction.

Figure 1

The negatively charged electron spectrum measured by PAMELA with two independent approaches: energy derived from the rigidity (full circles); energy derived from the calorimeter information (open circles). The error bars are statistical only.

Figure 2

The electron energy spectrum obtained in this work compared with modern measurements: CAPRICE94 [19], HEAT [20], AMS [21], MASS91 [22], Kobayashi [23], BETS [24], ATIC [25], HESS [26], Fermi [27]. Note that the data points from 23, 24, 25, 26, 27, indicated with blue symbols, and the highest data point from HEAT [20] are for the electron and positron sum.

Figure 3

Top: the $e^{-}$ spectrum obtained in this work compared with theoretical calculations. The solid line shows the standard GALPROP calculation [29] for a diffusion reacceleration model; the long-dashed line is a single power-law fit to the data above 30 GeV; the short-dashed line is a GALPROP calculation including a component from additional cosmic-ray electron sources. Bottom: the PAMELA positron fraction [28] compared with the previous GALPROP calculations with no (solid line) and with additional $e^{-}$ and $e^{+}$ components (short-dashed line). The error bars are statistical only and these were the only errors considered by the fitting procedures.