Measurement of separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope

TL;DR

The paper addresses separating cosmic-ray electron and positron fluxes in the $20$--$200$ GeV range using the Fermi LAT, which lacks an onboard magnet, by exploiting the Earth's geomagnetic shadow. It uses limb-pointed observations and two independent background-subtraction methods to extract $J(e^{+})$, $J(e^{-})$, and the positron fraction, confirming a rising fraction with energy up to $200$ GeV. The results provide the first absolute positron spectrum above $50$ GeV and extend measurements of the positron fraction beyond $100$ GeV, consistent with PAMELA. This magnet-free charge separation technique, validated against prior LAT results and with implications for pulsar/dark-matter scenarios, paves the way for future high-energy CR lepton studies and cross-checks with AMS-02.

Abstract

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 the Earth's shadow, which is offset in opposite directions for opposite charges due to the 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 GeV 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.

Figures (8)

• 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 $D_-<-2^\circ$. The spike at $D_+=0^\circ$ is due to atmospheric positrons. The events with $D_+>0^\circ$ are CR positrons plus residual proton background events. The events with $D_+<0^\circ$ 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 supplement 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 LATCRELATCRE1 (gray). Previous results form HEAT HEAT2001 and PAMELA Pamela2011CRE 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 HEAT1997Aguilar2007Pamela2. The Fermi statistical uncertainty is shown with error bars and the total (statistical plus systematic uncertainty) is shown as a shaded band.