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

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${}_{100}\ge 10^{-9}$\,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

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• A possible approach to three-dimensional cosmic-ray propagation in the Galaxy IV. Electrons and electron-induced gamma-rays
• Black Holes in our Galactic Halo: Compatibility with FGST and PAMELA Data and Constraints on the First Stars