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. 2016 Apr 15;116(15):151105.
doi: 10.1103/PhysRevLett.116.151105. Epub 2016 Apr 14.

Resolving the Extragalactic γ-Ray Background Above 50 GeV With the Fermi Large Area Telescope

M Ackermann  1 M Ajello  2 A Albert  3 W B Atwood  4 L Baldini  3   5 J Ballet  6 G Barbiellini  7   8 D Bastieri  9   10 K Bechtol  11 R Bellazzini  12 E Bissaldi  13 R D Blandford  3 E D Bloom  3 R Bonino  14   15 J Bregeon  16 R J Britto  17 P Bruel  18 R Buehler  1 G A Caliandro  3   19 R A Cameron  3 M Caragiulo  13   20 P A Caraveo  21 E Cavazzuti  22 C Cecchi  23   24 E Charles  3 A Chekhtman  25 J Chiang  3 G Chiaro  10 S Ciprini  22   23 J Cohen-Tanugi  16 L R Cominsky  26 F Costanza  13 S Cutini  22   23   27 F D'Ammando  28   29 A de Angelis  30 F de Palma  13   31 R Desiante  14   32 S W Digel  3 M Di Mauro  3 L Di Venere  13   20 A Domínguez  2 P S Drell  3 C Favuzzi  13   20 S J Fegan  18 E C Ferrara  33 A Franckowiak  3 Y Fukazawa  34 S Funk  35 P Fusco  13   20 F Gargano  13 D Gasparrini  22   23 N Giglietto  13   20 P Giommi  22 F Giordano  13   20 M Giroletti  28 G Godfrey  3 D Green  33   36 I A Grenier  6 S Guiriec  33   37 E Hays  33 D Horan  18 G Iafrate  7   38 T Jogler  3 G Jóhannesson  39 M Kuss  12 G La Mura  10   40 S Larsson  41   42 L Latronico  14 J Li  43 L Li  41   42 F Longo  7   8 F Loparco  13   20 B Lott  44 M N Lovellette  45 P Lubrano  23   24 G M Madejski  3 J Magill  36 S Maldera  14 A Manfreda  12 M Mayer  1 M N Mazziotta  13 P F Michelson  3 W Mitthumsiri  46 T Mizuno  47 A A Moiseev  36   48 M E Monzani  3 A Morselli  49 I V Moskalenko  3 S Murgia  50 M Negro  14   15 E Nuss  16 T Ohsugi  47 C Okada  34 N Omodei  3 E Orlando  3 J F Ormes  51 D Paneque  3   52 J S Perkins  33 M Pesce-Rollins  3   12 V Petrosian  3 F Piron  16 G Pivato  12 T A Porter  3 S Rainò  13   20 R Rando  9   10 M Razzano  12   53 S Razzaque  17 A Reimer  3   40 O Reimer  3   40 T Reposeur  44 R W Romani  3 M Sánchez-Conde  42   54 J Schmid  6 A Schulz  1 C Sgrò  12 D Simone  13 E J Siskind  55 F Spada  12 G Spandre  12 P Spinelli  13   20 D J Suson  56 H Takahashi  34 J B Thayer  3 L Tibaldo  57 D F Torres  43   58 E Troja  33   36 G Vianello  3 M Yassine  16 S Zimmer  42   54
Affiliations

Resolving the Extragalactic γ-Ray Background Above 50 GeV With the Fermi Large Area Telescope

M Ackermann et al. Phys Rev Lett. .

Abstract

The Fermi Large Area Telescope (LAT) Collaboration has recently released a catalog of 360 sources detected above 50 GeV (2FHL). This catalog was obtained using 80 months of data re-processed with Pass 8, the newest event-level analysis, which significantly improves the acceptance and angular resolution of the instrument. Most of the 2FHL sources at high Galactic latitude are blazars. Using detailed Monte Carlo simulations, we measure, for the first time, the source count distribution, dN/dS, of extragalactic γ-ray sources at E>50 GeV and find that it is compatible with a Euclidean distribution down to the lowest measured source flux in the 2FHL (∼8×10^{-12} ph cm^{-2} s^{-1}). We employ a one-point photon fluctuation analysis to constrain the behavior of dN/dS below the source detection threshold. Overall, the source count distribution is constrained over three decades in flux and found compatible with a broken power law with a break flux, S_{b}, in the range [8×10^{-12},1.5×10^{-11}] ph cm^{-2} s^{-1} and power-law indices below and above the break of α_{2}∈[1.60,1.75] and α_{1}=2.49±0.12, respectively. Integration of dN/dS shows that point sources account for at least 86_{-14}^{+16}% of the total extragalactic γ-ray background. The simple form of the derived source count distribution is consistent with a single population (i.e., blazars) dominating the source counts to the minimum flux explored by this analysis. We estimate the density of sources detectable in blind surveys that will be performed in the coming years by the Cherenkov Telescope Array.

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