1202.2856 (M. Ackermann et al.)

Anisotropies in the diffuse gamma-ray background measured by the Fermi
LAT    [PDF]

M. Ackermann, M. Ajello, A. Albert, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, E. D. Bloom, E. Bonamente, A. W. Borgland, T. J. Brandt, J. Bregeon, M. Brigida, P. Bruel, R. Buehler, S. Buson, G. A. Caliandro, R. A. Cameron, P. A. Caraveo, C. Cecchi, E. Charles, A. Chekhtman, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, J. Conrad, A. Cuoco, S. Cutini, F. D'Ammando, F. de Palma, C. D. Dermer, S. W. Digel, E. do Couto e Silva, P. S. Drell, A. Drlica-Wagner, R. Dubois, C. Favuzzi, S. J. Fegan, E. C. Ferrara, P. Fortin, Y. Fukazawa, P. Fusco, F. Gargano, D. Gasparrini, S. Germani, N. Giglietto, M. Giroletti, T. Glanzman, G. Godfrey, G. A. Gomez-Vargas, T. Grégoire, I. A. Grenier, J. E. Grove, S. Guiriec, M. Gustafsson, D. Hadasch, M. Hayashida, K. Hayashi, X. Hou, R. E. Hughes, G. Jóhannesson, A. S. Johnson, T. Kamae, J. Knödlseder, M. Kuss, J. Lande, L. Latronico, M. Lemoine-Goumard, T. Linden, A. M. Lionetto, M. Llena Garde, F. Longo, F. Loparco, M. N. Lovellette, P. Lubrano, M. N. Mazziotta, J. E. McEnery, W. Mitthumsiri, T. Mizuno, C. Monte, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, M. Naumann-Godo, J. P. Norris, E. Nuss, T. Ohsugi, A. Okumura, M. Orienti, E. Orlando, J. F. Ormes, D. Paneque, J. H. Panetta, D. Parent, V. Pavlidou, M. Pesce-Rollins, M. Pierbattista, F. Piron, G. Pivato, S. Rainò, R. Rando, A. Reimer, O. Reimer, M. Roth, C. Sbarra, J. Schmitt, C. Sgrò, J. Siegal-Gaskins, E. J. Siskind, G. Spandre, P. Spinelli, A. W. Strong, D. J. Suson, H. Takahashi, T. Tanaka, J. B. Thayer, L. Tibaldo, M. Tinivella, D. F. Torres, G. Tosti, E. Troja, T. L. Usher, J. Vandenbroucke, V. Vasileiou, G. Vianello, V. Vitale, A. P. Waite, B. L. Winer, K. S. Wood, M. Wood, Z. Yang, S. Zimmer, for the Fermi LAT Collaboration, E. Komatsu

The contribution of unresolved sources to the diffuse gamma-ray background
could induce anisotropies in this emission on small angular scales. We analyze
the angular power spectrum of the diffuse emission measured by the Fermi LAT at
Galactic latitudes |b| > 30 deg in four energy bins spanning 1 to 50 GeV. At
multipoles \ell \ge 155, corresponding to angular scales \lesssim 2 deg,
angular power above the photon noise level is detected at >99.99% CL in the 1-2
GeV, 2-5 GeV, and 5-10 GeV energy bins, and at >99% CL at 10-50 GeV. Within
each energy bin the measured angular power takes approximately the same value
at all multipoles \ell \ge 155, suggesting that it originates from the
contribution of one or more unclustered source populations. The amplitude of
the angular power normalized to the mean intensity in each energy bin is
consistent with a constant value at all energies, C_P/^2 = 9.05 +/- 0.84 x
10^{-6} sr, while the energy dependence of C_P is consistent with the
anisotropy arising from one or more source populations with power-law photon
spectra with spectral index \Gamma_s = 2.40 +/- 0.07. We discuss the
implications of the measured angular power for gamma-ray source populations
that may provide a contribution to the diffuse gamma-ray background.

View original: http://arxiv.org/abs/1202.2856