Unveiling the Gamma-ray Source Count Distribution Below the Fermi Detection Limit with Photon Statistics

Abstract

The source-count distribution as a function of their flux, dN/dS, is one of the main quantities characterizing gamma-ray source populations. 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|>30 deg) between 1 GeV 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-year 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 of index between 1.9 and 2.0. We measure dN/dS down to an integral flux of ~2x10^{-11} cm^{-2}s^{-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}x10^{-8} cm^{-2}s^{-1}. The power-law index n_1=3.1^{+0.7}_{-0.5} for bright sources above the 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 6.4x10^{-11} cm^{-2}s^{-1} at 95% confidence level. The high-latitude gamma-ray sky between 1 GeV and 10 GeV is shown to be composed of ~25% point sources, ~69.3% diffuse Galactic foreground emission, and ~6% isotropic diffuse background.

28 pages, 20 figures, 6 tables; v2: minor changes and additions, accepted for publication in ApJS

References in corpus (27)

• Big-Bang Nucleosynthesis
• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• The spectrum of isotropic diffuse gamma-ray emission between 100 MeV and 820 GeV
• Strong support for the millisecond pulsar origin of the Galactic center GeV excess
• The Spectrum and Morphology of the Fermi Bubbles
• 2FHL: The Second Catalog of Hard Fermi-LAT Sources
• Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy
• The Fermi-LAT high-latitude Survey: Source Count Distributions and the Origin of the Extragalactic Diffuse Background
• The Origin of the Extragalactic Gamma-Ray Background and Implications for Dark-Matter Annihilation
• Star-forming galaxies as the origin of diffuse high-energy backgrounds: Gamma-ray and neutrino connections, and implications for starburst history
• The Starburst Contribution to the Extra-Galactic Gamma-Ray Background
• The nature of the Diffuse Gamma-Ray Background
• Dark matter annihilation or unresolved astrophysical sources? Anisotropy probe of the origin of cosmic gamma-ray background
• Composition of the Fermi-LAT isotropic gamma-ray background intensity: Emission from extragalactic point sources and dark matter annihilations
• Cosmic Gamma-Ray Background from Star-Forming Galaxies
• Joint anisotropy and source count constraints on the contribution of blazars to the diffuse gamma-ray background
• Diffuse gamma-ray emission from galactic pulsars
• Angular Momentum Exchange by Gravitational Torques and Infall in the Circumbinary Disk of the Protostellar System L1551 NE
• A Spitzer Five-Band Analysis of the Jupiter-Sized Planet TrES-1
• Active-Region Tilt Angles: Magnetic Versus White-Light Determinations of Joy's Law
• Dark matter searches in the gamma-ray extragalactic background via cross-correlations with galaxy catalogues
• The Gamma-Ray-Flux Probability Distribution Function from Galactic Halo Substructure
• Distinguishing Dark Matter from Unresolved Point Sources in the Inner Galaxy with Photon Statistics
• The Deep Diffuse Extragalactic Radio Sky at 1.75 GHz
• Fermi-LAT gamma-ray anisotropy and intensity explained by unresolved Radio-Loud Active Galactic Nuclei
• Constraining Dark Matter in Galactic Substructure
• A Determination of the Gamma-ray Flux and Photon Spectral Index Distributions of Blazars from the Fermi-LAT 3LAC

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• A Search for Neutrino Point-Source Populations in 7 Years of IceCube Data with Neutrino-count Statistics
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• The Galactic Isotropic $\gamma$-ray Background and Implications for Dark Matter
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