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

Abstract: 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.

• The paper presents key models—including Galactic, pulsar, and dark matter scenarios—to explain the CRE spectrum observed between 20 GeV and 1 TeV.
• The authors compare traditional supernova remnant diffusion against pulsar contributions, addressing challenges posed by the PAMELA positron fraction.
• By analyzing dark matter-induced leptonic channels and associated boost factors, the study calls for further observations to resolve competing hypotheses.

Interpretation of the High Energy Electron-Positron Spectrum Measured by Fermi-LAT

The paper provides an in-depth analysis of cosmic-ray electrons-plus-positrons (CRE) spectra measured by the Fermi Large Area Telescope (Fermi-LAT), focusing on the energy range of 20 GeV to 1 TeV. The authors explore multiple interpretations to account for the observed spectral characteristics, with emphasis on the phenomena occurring at high energies. These interpretations include both traditional models of Galactic CRE and models involving additional sources, such as pulsars and dark matter (DM).

Summary of Key Interpretations

1. Galactic Cosmic Ray Electron Model (GCRE):
   • The GCRE model posits that electrons and positrons are continuously produced by astrophysical sources, predominantly supernova remnants (SNR), dispersed across the Galactic disk.
   • Various parameter settings within this traditional diffusion model provide a reasonable fit to the Fermi-LAT measurements. However, these configurations struggle to reconcile with the positron fraction reported by the PAMELA experiment, especially considering the steep electron spectrum revealed by Fermi-LAT.
2. Pulsar Contribution:
   • The paper considers scenarios where mature pulsars, in addition to the GCRE, contribute significant CRE flux through their electron-positron pair production.
   • Pulsars, with mature ages and proximity within several hundred parsecs, are evaluated as potential sources explaining the hard spectrum observed by Fermi-LAT and the rising positron fraction from PAMELA.
3. Dark Matter Annihilation or Decay:
   • An alternative explanation involves the annihilation or decay of dark matter particles in the Galactic halo. This hypothesis is investigated under the assumption that the DM-derived electrons and positrons could account for the observed excesses.
   • The study explores various DM models, focusing on those producing leptonic channels to evade constraints from antiproton measurements. The analysis concludes that models with DM masses between 0.4 and 2 TeV are feasible, requiring significant enhancements in annihilation rates ("boost factors").

Implications and Observations

• Observational Consistency: The analysis underlines that both pulsar-origin models and certain classes of DM interpretations can effectively account for the CRE spectrum measured by Fermi-LAT alongside PAMELA's positron fraction. Yet, the distinctions between these scenarios become blurry, suggesting observational constraints or novel signatures are necessary for unequivocal discrimination.
• Astrophysical vs. Exotic Origins: While the pulsar contribution is physically plausible, the paper carefully evaluates the dark matter interpretation in the context of the annihilation cross-section, considering the constraints from gamma-ray emissions and cosmic ray anisotropies. Notably, pulsar models appear favored due to the lack of necessity for a non-standard DM interpretation when conventional astrophysical sources suffice.
• Future Observational Prospects: The authors advocate for further observational efforts, indicating that upcoming CRE anisotropy measurements and lower-energy range data could be crucial in distinguishing between these alternatives. Experiments like AMS-02 and parameters such as the CRE spectral shape above hundreds of GeV may potentially discern pulsar-induced features from those stemming from dark matter interactions.

Conclusion

The paper comprehensively explores the potential interpretations of high-energy CRE data observed by Fermi-LAT. While high-fidelity pulsar models and strategic DM setups both present viable models accommodating current measurements, they also underscore the complexity and overlapping nature of astrophysical phenomena and possible new physics. The paper casts light on the necessity of continued experimental scrutiny and model refinement for resolving these competing hypotheses, propelling future research avenues in cosmic ray physics and astroparticle studies.