Inferred Cosmic-Ray Spectrum from Fermi Large Area Telescope γ-Ray Observations of Earth’s Limb

M. Ackermann et al. (Fermi LAT Collaboration)
Phys. Rev. Lett. 112, 151103 – Published 17 April 2014

Abstract

Recent accurate measurements of cosmic-ray (CR) species by ATIC-2, CREAM, and PAMELA reveal an unexpected hardening in the proton and He spectra above a few hundred GeV, a gradual softening of the spectra just below a few hundred GeV, and a harder spectrum of He compared to that of protons. These newly discovered features may offer a clue to the origin of high-energy CRs. We use the Fermi Large Area Telescope observations of the γ-ray emission from Earth’s limb for an indirect measurement of the local spectrum of CR protons in the energy range 90GeV6TeV (derived from a photon energy range 15 GeV–1 TeV). Our analysis shows that single power law and broken power law spectra fit the data equally well and yield a proton spectrum with index 2.68±0.04 and 2.61±0.08 above 200GeV, respectively.

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  • Received 13 December 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.151103

© 2014 American Physical Society

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Vol. 112, Iss. 15 — 18 April 2014

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