Journal
PHYSICAL REVIEW LETTERS
Volume 112, Issue 15, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevLett.112.151103
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Funding
- NASA [NNX11AQ06 G, NNX13AC47 G]
- K.A. Wallenberg Foundation
- Italian Ministry of Education, University and Research (MIUR) [FIRB-2012-RBFR12PM1 F]
- ICREA Funding Source: Custom
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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 gamma-ray emission from Earth's limb for an indirect measurement of the local spectrum of CR protons in the energy range similar to 90 GeV-6 TeV (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 similar to 200 GeV, respectively.
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