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Inferred cosmic-ray spectrum from Fermi large area telescope γ-ray observations of Earth's limb.

  • Author(s): Ackermann, M;
  • Ajello, M;
  • Albert, A;
  • Allafort, A;
  • Baldini, L;
  • Barbiellini, G;
  • Bastieri, D;
  • Bechtol, K;
  • Bellazzini, R;
  • Blandford, RD;
  • Bloom, ED;
  • Bonamente, E;
  • Bottacini, E;
  • Bouvier, A;
  • Brandt, TJ;
  • Brigida, M;
  • Bruel, P;
  • Buehler, R;
  • Buson, S;
  • Caliandro, GA;
  • Cameron, RA;
  • Caraveo, PA;
  • Cecchi, C;
  • Charles, E;
  • Chaves, RCG;
  • Chekhtman, A;
  • Chiang, J;
  • Chiaro, G;
  • Ciprini, S;
  • Claus, R;
  • Cohen-Tanugi, J;
  • Conrad, J;
  • Cutini, S;
  • Dalton, M;
  • D'Ammando, F;
  • de Angelis, A;
  • de Palma, F;
  • Dermer, CD;
  • Digel, SW;
  • Di Venere, L;
  • do Couto e Silva, E;
  • Drell, PS;
  • Drlica-Wagner, A;
  • Favuzzi, C;
  • Fegan, SJ;
  • Ferrara, EC;
  • Focke, WB;
  • Franckowiak, A;
  • Fukazawa, Y;
  • Funk, S;
  • Fusco, P;
  • Gargano, F;
  • Gasparrini, D;
  • Germani, S;
  • Giglietto, N;
  • Giordano, F;
  • Giroletti, M;
  • Glanzman, T;
  • Godfrey, G;
  • Gomez-Vargas, GA;
  • Grenier, IA;
  • Grove, JE;
  • Guiriec, S;
  • Gustafsson, M;
  • Hadasch, D;
  • Hanabata, Y;
  • Harding, AK;
  • Hayashida, M;
  • Hayashi, K;
  • Hewitt, JW;
  • Horan, D;
  • Hou, X;
  • Hughes, RE;
  • Inoue, Y;
  • Jackson, MS;
  • Jogler, T;
  • Jóhannesson, G;
  • Johnson, AS;
  • Kamae, T;
  • Kawano, T;
  • Knödlseder, J;
  • Kuss, M;
  • Lande, J;
  • Larsson, S;
  • Latronico, L;
  • Longo, F;
  • Loparco, F;
  • Lovellette, MN;
  • Lubrano, P;
  • Mayer, M;
  • Mazziotta, MN;
  • McEnery, JE;
  • Mehault, J;
  • Michelson, PF;
  • Mitthumsiri, W;
  • Mizuno, T;
  • Moiseev, AA;
  • Monte, C;
  • Monzani, ME;
  • Morselli, A;
  • Moskalenko, IV;
  • Murgia, S;
  • Nemmen, R;
  • Nuss, E;
  • Ohsugi, T;
  • Okumura, A;
  • Orienti, M;
  • Orlando, E;
  • Ormes, JF;
  • Paneque, D;
  • Panetta, JH;
  • Perkins, JS;
  • Pesce-Rollins, M;
  • Piron, F;
  • Pivato, G;
  • Porter, TA;
  • Rainò, S;
  • Rando, R;
  • Razzano, M;
  • Razzaque, S;
  • Reimer, A;
  • Reimer, O;
  • Ritz, S;
  • Roth, M;
  • Schaal, M;
  • Schulz, A;
  • Sgrò, C;
  • Siskind, EJ;
  • Spandre, G;
  • Spinelli, P;
  • Strong, AW;
  • Takahashi, H;
  • Takeuchi, Y;
  • Thayer, JG;
  • Thayer, JB;
  • Thompson, DJ;
  • Tibaldo, L;
  • Tinivella, M;
  • Torres, DF;
  • Tosti, G;
  • Troja, E;
  • Tronconi, V;
  • Usher, TL;
  • Vandenbroucke, J;
  • Vasileiou, V;
  • Vianello, G;
  • Vitale, V;
  • Werner, M;
  • Winer, BL;
  • Wood, KS;
  • Wood, M;
  • Yang, Z;
  • Fermi LAT Collaboration
  • et al.
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 ∼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 ∼200  GeV, respectively.

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