Matt Kistler homepage. Matthew Kistler New Constraints on the Highest-Energy Cosmic-Ray Electrons and Positrons
What is the highest energy reached by cosmic-ray electrons and positrons?
Interest in the cosmic-ray electron spectrum at Earth is at an all-time high, arising from both astrophysical and dark matter-related concerns. In the GeV energy range, much greater clarity than what had existed in preceding decades has been brought by the PAMELA and Fermi satellites. These measurements, which observe electrons and positrons directly, become difficult above ~1 TeV due to their fixed detector areas relative to declining particle fluxes. Using the indirect technique of observing atmospheric air showers, HESS has pushed the energy frontier up to several TeV, although above this, no electron measurements have been reported. In this paper with Hasan Yuksel, we discuss how the similarity of the electromagnetic showers produced in the atmosphere by energetic electrons and gamma rays allow us to use published limits on isotropic gamma-ray fluxes to constrain the electron+positron spectrum up to >PeV energies. Moreover, we show that recent discoveries of gamma-ray sources nearby in the Galaxy can lead to large positron fluxes in this range, where none had been previously expected. We conclude that it would be surprising if the influence of a nearby pulsar was not present in cosmic-ray positron measurements.
