The Fermi-LAT high-latitude Survey: Source Count Distributions and the Origin of the Extragalactic Diffuse Background

LAT Collaboration

doi:10.1088/0004-637X/720/1/435

paper

The Fermi-LAT high-latitude Survey: Source Count Distributions and the Origin of the Extragalactic Diffuse Background

arXiv:1003.0895 · doi:10.1088/0004-637X/720/1/435

Abstract

This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the {\it Fermi}-LAT survey. In this work we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, {\it Fermi} sources are on average steeper than previously found (e.g. in the bright source list) with an average photon index of 2.40 $\pm 0.02$ over the entire 0.1--100\,GeV energy band. We confirm that FSRQs have steeper spectra than BL Lac objects with an average index of 2.48 $\pm 0.02$ versus 2.18 $\pm 0.02$ . Using several methods we build the deepest source count distribution at GeV energies deriving that the intrinsic source (i.e. blazar) surface density at F $_{100} \geq 1 0^{- 9}$ \,ph cm $^{- 2}$ s $^{- 1}$ is 0.12 $_{- 0.02}^{+ 0.03}$ \,deg $^{- 2}$ . The integration of the source count distribution yields that point sources contribute 16 $(\pm 1.8)$ \,\% ( $\pm$ 7\,\% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT we can rule out the hypothesis that point-like sources (i.e. blazars) produce a larger fraction of the diffuse emission.

Version replaced to match the published one. Contact authors: M. Ajello and A. Tramacere

The Fermi-LAT high-latitude Survey: Source Count Distributions and the Origin of the Extragalactic Diffuse Background

Abstract

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