Inferred Cosmic-Ray Spectrum from Fermi Large Area Telescope<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi></mml:math>-Ray Observations of Earth’s Limb

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 $\ensuremath{\gamma}$-ray emission from Earth's limb for an indirect measurement of the local spectrum of CR protons in the energy range $\ensuremath{\sim}90\text{ }\text{ }\mathrm{GeV}--6\text{ }\text{ }\mathrm{TeV}$ (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 $2.68\ifmmode\pm\else\textpm\fi{}0.04$ and $2.61\ifmmode\pm\else\textpm\fi{}0.08$ above $\ensuremath{\sim}200\text{ }\text{ }\mathrm{GeV}$, respectively.