The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV

M. Ackermann , M. Ajello , A. Albert , W.B. Atwood , L. Baldini , J. Ballet (1) , G. Barbiellini , D. Bastieri , K. Bechtol , R. Bellazzini , E. Bissaldi , R.D. Blandford , E.D. Bloom , E. Bottacini , T.J. Brandt , J. Bregeon (2) , P. Bruel (3) , R. Buehler , S. Buson , G.A. Caliandro , R.A. Cameron , M. Caragiulo , P.A. Caraveo , E. Cavazzuti , C. Cecchi , E. Charles , A. Chekhtman , J. Chiang , G. Chiaro , S. Ciprini , R. Claus , J. Cohen-Tanugi (4) , J. Conrad , A. Cuoco , S. Cutini , F. d'Ammando , A. de Angelis , F. de Palma , C.D. Dermer , S.W. Digel , E. Do Couto E Silva , P.S. Drell , C. Favuzzi , E.C. Ferrara , W.B. Focke , A. Franckowiak , Y. Fukazawa , S. Funk , P. Fusco , F. Gargano , D. Gasparrini , S. Germani , N. Giglietto , P. Giommi , F. Giordano , M. Giroletti , G. Godfrey , G.A. Gomez-Vargas , I.A. Grenier (1) , S. Guiriec , M. Gustafsson , D. Hadasch , K. Hayashi , E. Hays , J.W. Hewitt , P. Ippoliti , T. Jogler , G. Jóhannesson , A.S. Johnson , W.N. Johnson , T. Kamae , J. Kataoka , J. Knödlseder (5) , M. Kuss , S. Larsson , L. Latronico , J. Li , L. Li , F. Longo , F. Loparco , B. Lott , M.N. Lovellette , P. Lubrano , G.M. Madejski , A. Manfreda , F. Massaro , M. Mayer , M.N. Mazziotta , J.E. Mcenery , P.F. Michelson , W. Mitthumsiri , T. Mizuno , A.A. Moiseev , M.E. Monzani , A. Morselli , I.V. Moskalenko , S. Murgia , R. Nemmen , E. Nuss (2) , T. Ohsugi , N. Omodei , E. Orlando , J.F. Ormes , D. Paneque , J.H. Panetta , J.S. Perkins , M. Pesce-Rollins , F. Piron (2) , G. Pivato , T.A. Porter , S. Rainò , R. Rando , M. Razzano , S. Razzaque , A. Reimer , O. Reimer , T. Reposeur (6) , S. Ritz , R.W. Romani , M. Sánchez-Conde , M. Schaal , A. Schulz , C. Sgrò , E.J. Siskind , G. Spandre , P. Spinelli , A.W. Strong , D.J. Suson , H. Takahashi , J.G. Thayer , J.B. Thayer , L. Tibaldo , M. Tinivella , D.F. Torres , G. Tosti , E. Troja , Y. Uchiyama , G. Vianello , M. Werner , B.L. Winer , K.S. Wood , M. Wood , G. Zaharijas , S. Zimmer

1 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
2 LUPM - Laboratoire Univers et Particules de Montpellier
3 LLR - Laboratoire Leprince-Ringuet
4 UM2 - Université Montpellier 2 - Sciences et Techniques
5 IRAP - Institut de recherche en astrophysique et planétologie
6 CNRS - Centre National de la Recherche Scientifique

The γ-ray sky can be decomposed into individually detected sources, diffuse emission attributed to the interactions of Galactic cosmic rays with gas and radiation fields, and a residual all-sky emission component commonly called the isotropic diffuse γ-ray background (IGRB). The IGRB comprises all extragalactic emissions too faint or too diffuse to be resolved in a given survey, as well as any residual Galactic foregrounds that are approximately isotropic. The first IGRB measurement with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) used 10 months of sky-survey data and considered an energy range between 200 MeV and 100 GeV. Improvements in event selection and characterization of cosmic-ray backgrounds, better understanding of the diffuse Galactic emission (DGE), and a longer data accumulation of 50 months allow for a refinement and extension of the IGRB measurement with the LAT, now covering the energy range from 100 MeV to 820 GeV. The IGRB spectrum shows a significant high-energy cutoff feature and can be well described over nearly four decades in energy by a power law with exponential cutoff having a spectral index of 2.32 ± 0.02 and a break energy of (279 ± 52) GeV using our baseline DGE model. The total intensity attributed to the IGRB is (7.2 ± 0.6) × 10(–)(6) cm(–)(2) s(–)(1) sr(–)(1) above 100 MeV, with an additional +15%/–30% systematic uncertainty due to the Galactic diffuse foregrounds.

Mots clés: gamma rays: diffuse background; diffuse radiation; gamma ray: energy spectrum; gamma ray: background; cosmic radiation: galaxy; gamma ray: VHE; GLAST; direct detection; cosmic background radiation; gamma ray: emission; gas

Domaines: Astrophysique [astro-ph]

M. Ackermann, M. Ajello, A. Albert, W.B. Atwood, L. Baldini, et al.. The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV. Astrophys.J., 2015, 799, pp.86. ⟨10.1088/0004-637X/799/1/86⟩. ⟨hal-02017424⟩