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Title: Unveiling the Gamma-Ray Source Count Distribution Below the Fermi Detection Limit with Photon Statistics

The source-count distribution as a function of their flux, dN/dS, is one of the main quantities characterizing gamma-ray source populations. In this paper, we employ statistical properties of the Fermi Large Area Telescope (LAT) photon counts map to measure the composition of the extragalactic gamma-ray sky at high latitudes (|b| greater-than or slanted equal to 30°) between 1 and 10 GeV. We present a new method, generalizing the use of standard pixel-count statistics, to decompose the total observed gamma-ray emission into (a) point-source contributions, (b) the Galactic foreground contribution, and (c) a truly diffuse isotropic background contribution. Using the 6 yr Fermi-LAT data set (P7REP), we show that the dN/dS distribution in the regime of so far undetected point sources can be consistently described with a power law with an index between 1.9 and 2.0. We measure dN/dS down to an integral flux of ~2 x 10 -11cm -2s -1, improving beyond the 3FGL catalog detection limit by about one order of magnitude. The overall dN/dS distribution is consistent with a broken power law, with a break at 2.1 +1.0 -1.3 x 10 -8cm -2s -1. The power-law index n 1 = 3.1 +0.7 -0.5 for bright sources above themore » break hardens to n 2 = 1.97 ± 0.03 for fainter sources below the break. A possible second break of the dN/dS distribution is constrained to be at fluxes below 6.4 x 10 -11cm -2s -1 at 95% confidence level. Finally, the high-latitude gamma-ray sky between 1 and 10 GeV is shown to be composed of ~25% point sources, ~69.3% diffuse Galactic foreground emission, and ~6% isotropic diffuse background.« less
ORCiD logo [1] ;  [2] ; ORCiD logo [1] ; ORCiD logo [1] ;  [3]
  1. Univ. of Turin (Italy). Dept. of Physics; National Inst. for Nuclear Physics (INFN), Turin (Italy)
  2. National Inst. for Nuclear Physics (INFN), Turin (Italy); RWTH Aachen Univ. (Germany). Inst. for Theoretical Particle Physics and Cosmology (TTK)
  3. Technical Univ. of Munich, Garching (Germany). Physics Dept.
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
The Astrophysical Journal. Supplement Series (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal. Supplement Series (Online); Journal Volume: 225; Journal Issue: 2; Journal ID: ISSN 1538-4365
American Astronomical Society/IOP
Research Org:
National Inst. for Nuclear Physics (INFN), Turin (Italy); Univ. of Turin (Italy); Technical Univ. of Munich, Garching (Germany)
Sponsoring Org:
USDOE; Ministry of Education, University and Research (MIUR) (Italy); National Inst. for Nuclear Physics (INFN) (Italy); Univ. of Turin (Italy); Compagnia di San Paolo (Italy); German-Israeli Foundation for Scientific Research and Development (GIF) (Israel)
Contributing Orgs:
Fermi-LAT Collaboration; RWTH Aachen Univ. (Germany)
Country of Publication:
United States
79 ASTRONOMY AND ASTROPHYSICS; diffuse background; gamma-rays; statistical methods
OSTI Identifier: