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

Journal Article · · The Astrophysical Journal. Supplement Series (Online)
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.
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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 n1 = 3.1+0.7-0.5 for bright sources above the break hardens to n2 = 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.

Research Organization:
National Inst. for Nuclear Physics (INFN), Turin (Italy); Univ. of Turin (Italy); Technical Univ. of Munich, Garching (Germany)
Sponsoring Organization:
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 Organization:
Fermi-LAT Collaboration; RWTH Aachen Univ. (Germany)
Grant/Contract Number:
2012CPPYP7
OSTI ID:
1355703
Journal Information:
The Astrophysical Journal. Supplement Series (Online), Vol. 225, Issue 2; ISSN 1538-4365
Publisher:
American Astronomical Society/IOPCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 38 works
Citation information provided by
Web of Science

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Cited By (7)

The Fermi-LAT GeV excess as a tracer of stellar mass in the Galactic bulge journal August 2018
The galactic isotropic γ-ray background and implications for dark matter journal June 2018
Evidence against Star-forming Galaxies as the Dominant Source of Icecube Neutrinos journal February 2017
Deriving the Contribution of Blazars to the Fermi -LAT Extragalactic γ -ray Background at E > 10 GeV with Efficiency Corrections and Photon Statistics journal March 2018
Unresolved Gamma-Ray Sky through its Angular Power Spectrum text January 2018
Evidence against star-forming galaxies as the dominant source of IceCube neutrinos text January 2015
Unveiling dark matter with cosmic messengers text January 2020

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