Measurement of the Cosmic Ray $e^{+}+e^{-}$ Spectrum from 20 GeV to 1 TeV with the Fermi Large Area Telescope

Designed as a high-sensitivity gamma-ray observatory, the Fermi Large Area Telescope is also an electron detector with a large acceptance exceeding $2{\mathrm{m}}^2\mathrm{sr}$ at 300 GeV. Building on the gamma-ray analysis, we have developed an efficient electron detection strategy which provides sufficient background rejection for measurement of the steeply falling electron spectrum up to 1 TeV. Our high precision data show that the electron spectrum falls with energy as $E^{-3.0}$ and does not exhibit prominent spectral features. Interpretations in terms of a conventional diffusive model as well as a potential local extra component are briefly discussed.

Figure 1

Energy resolution for the LAT after electron selection; the full widths of the smallest energy window containing the 68% and the 95% of the energy dispersion distribution are shown. The comparison with beam test data up to 282 GeV and for on-axis and at 60° incidence shown in the figure indicates good agreement with the resolution estimated from the simulation.

Figure 2

Distribution of the transverse sizes of the showers (above 150 GeV) in the CAL at an intermediate stage of the selection, where a large contamination from protons is still visible. Flight data (black points) and MC simulation (gray solid line) show very good agreement; the underlying distributions of electron and hadron samples are visible in the left (red) and the right (blue) peaks, respectively.

Figure 3

The Fermi LAT CR electron spectrum (red filled circles). Systematic errors are shown by the gray band. The two-headed arrow in the top-right corner of the figure gives size and direction of the rigid shift of the spectrum implied by a shift of $\genfrac{}{}{0ex}{}{+5\%}{-10\%}$ of the absolute energy, corresponding to the present estimate of the uncertainty of the LAT energy scale. Other high-energy measurements and a conventional diffusive model [1] are shown.