Measurement of Separate Cosmic-Ray Electron and Positron Spectra with the Fermi Large Area Telescope

We measured separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Because the instrument does not have an onboard magnet, we distinguish the two species by exploiting Earth’s shadow, which is offset in opposite directions for opposite charges due to Earth’s magnetic field. We estimate and subtract the cosmic-ray proton background using two different methods that produce consistent results. We report the electron-only spectrum, the positron-only spectrum, and the positron fraction between 20 and 200 GeV. We confirm that the fraction rises with energy in the 20–100 GeV range. The three new spectral points between 100 and 200 GeV are consistent with a fraction that is continuing to rise with energy.

DOI: http://dx.doi.org/10.1103/PhysRevLett.108.011103

Figure 1

Examples of calculated electron (red) and positron (blue) trajectories arriving at the detector, for 28 GeV particles arriving within the Equatorial plane (viewed from the North pole). Forbidden trajectories are solid and allowed trajectories are dashed. Inset: the three selection regions (electron-only, positron-only, and both-allowed) for the same particle energy and spacecraft position as the trajectory traces (viewed from the instrument position in the Equatorial plane).

Figure 2

Distribution of D₊, for events with D_-<-2°. The spike at D₊=0° is due to atmospheric positrons. The events with D₊>0° are CR positrons plus residual proton background events. The events with D₊<0° are residual background events.

Figure 3

Transverse shower size distribution in the electron-only region. In the positron-only region, the number of events with small transverse shower size is smaller, but the mean and width of the distribution are similar. See [39] for additional figures.

Figure 4

Energy spectra for e⁺, e^-, and e⁺+e^- (control region). In the control region where both species are allowed, this analysis reproduces the Fermi LAT results reported previously for the total electron plus positron spectrum [20, 21] (gray). Previous results form HEAT [9] and PAMELA [40] are shown for reference. The bottom panel shows that the ratio between the sum and the control flux is consistent with 1 as expected.

Figure 5

Positron fraction measured by the Fermi LAT and by other experiments [7, 14, 16]. The Fermi statistical uncertainty is shown with error bars and the total (statistical plus systematic uncertainty) is shown as a shaded band.