On possible interpretations of the high energy electron-positron spectrum measured by the Fermi Large Area Telescope

The Fermi-LAT experiment recently reported high precision measurements of the spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20 GeV and 1 TeV. The spectrum shows no prominent spectral features, and is significantly harder than that inferred from several previous experiments. Here we discuss several interpretations of the Fermi results based either on a single large scale Galactic CRE component or by invoking additional electron-positron primary sources, e.g. nearby pulsars or particle Dark Matter annihilation. We show that while the reported Fermi-LAT data alone can be interpreted in terms of a single component scenario, when combined with other complementary experimental results, specifically the CRE spectrum measured by H.E.S.S. and especially the positron fraction reported by PAMELA between 1 and 100 GeV, that class of models fails to provide a consistent interpretation. Rather, we find that several combinations of parameters, involving both the pulsar and dark matter scenarios, allow a consistent description of those results. We also briefly discuss the possibility of discriminating between the pulsar and dark matter interpretations by looking for a possible anisotropy in the CRE flux.

💡 Research Summary

The paper investigates how to interpret the high‑precision cosmic‑ray electron‑plus‑positron (CRE) spectrum measured by the Fermi Large Area Telescope (LAT) in the 20 GeV–1 TeV range, together with complementary data from H.E.S.S. (high‑energy electrons) and PAMELA (positron fraction 1–100 GeV). The authors first examine whether a conventional Galactic cosmic‑ray electron (GCRE) model, in which electrons are injected continuously by a smooth distribution of astrophysical sources (e.g., supernova remnants) and then propagate through diffusion, energy losses, and possible re‑acceleration, can account for the observations. Using the GALPROP code, three baseline models are considered: model 0 (γ₀ = 2.54, δ = 0.33, D₀ = 3.6 × 10²⁸ cm² s⁻¹), model 1 (γ₀ = 2.42, δ = 0.33) and model 2 (γ₀ = 2.33, δ = 0.60). Models 1 and 2 provide a good fit to the Fermi‑LAT CRE spectrum alone, but all three fail to reproduce the low‑energy electron data (AMS‑01, HEAT) and, crucially, cannot explain the rising positron fraction reported by PAMELA. This indicates that a single, smooth Galactic component is insufficient when the full set of measurements is considered.

The authors then explore two classes of additional primary sources. First, nearby mature pulsars (e.g., Geminga, Monogem) are modeled as burst‑like injectors of electron‑positron pairs with a power‑law spectrum (index ≈ 1.5–2.0) and total energy 10⁴⁸–10⁵⁰ erg. Accounting for their distances (∼0.2–1 kpc) and ages (∼10⁵ yr) and the subsequent energy‑dependent propagation, the pulsar contribution can simultaneously flatten the Fermi‑LAT spectrum around 100 GeV, produce the steepening above ∼400 GeV observed by H.E.S.S., and generate the PAMELA positron excess. The parameter space is constrained by the need to avoid overshooting the total CRE flux while matching the positron fraction shape.

Second, dark‑matter (DM) annihilation or decay is considered. Representative models with particle mass mχ ≈ 1 TeV and thermally‑averaged cross‑section ⟨σv⟩ ≈ 10⁻²⁶ cm³ s⁻¹, annihilating predominantly into leptonic final states (μ⁺μ⁻ or τ⁺τ⁻), are shown to produce a hard primary e⁺e⁻ spectrum that can explain both the PAMELA rise and the high‑energy CRE data. The spatial distribution of DM (smooth halo) leads to negligible anisotropy in the CRE flux, in contrast to the pulsar scenario where a dipole anisotropy at the percent level is expected because of the dominant contribution from a nearby source.

The paper discusses how future observations could discriminate between the two scenarios. A detection of a directional anisotropy in the CRE flux with Fermi‑LAT or the upcoming Cherenkov Telescope Array (CTA) would favor the pulsar hypothesis, whereas the absence of anisotropy combined with possible gamma‑ray line signatures or associated neutrino fluxes would support a DM origin. The authors also note that the required adjustments to the electron injection index in the conventional models have only modest impact on the predicted diffuse gamma‑ray emission, preserving consistency with existing Fermi‑LAT gamma‑ray observations.

In conclusion, while a single large‑scale Galactic electron component can fit the Fermi‑LAT data in isolation, it cannot accommodate the full set of CRE and positron measurements. Introducing either a population of nearby pulsars or a leptophilic dark‑matter component yields viable, self‑consistent explanations. Ongoing and future high‑precision measurements of CRE spectra, positron fractions, and anisotropies will be essential to distinguish between these competing interpretations.