Author(s): A. Abdo A., Ackermann M., Ajello M., Antolini E., Baldini L., Ballet J., Barbiellini G., Bastieri D., M. Baughman B., Bechtol K., Bellazzini R., Berenji B., Bruel P., M. Casandjian J., Cohen-Tanugi J., Dumora D., Farnier C., J. Fegan S., A. Grenier I., Guiriec Sylvain, Horan D., Knodlseder J., Lemoine-Goumard M., Lott B., Nuss E., Parent D., Pelassa V., Piron F., Tibaldo L., Vilchez N.
(Article) Published:
The Astrophysical Journal / The Astrophysical Journal Letters, vol. 720 p.435-453 (2010)
Links openAccess full text : 
Ref HAL: in2p3-00530044_v1
Ref Arxiv: 1003.0895
DOI: 10.1088/0004-637X/720/1/435
Ref. & Cit.: NASA ADS
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161 citations
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$\pm0.02$ 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$\pm0.02$ versus 2.18$\pm0.02$. 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$_{100}\geq10^{-9}$\,ph cm$^{-2}$ s$^{-1}$ is 0.12$^{+0.03}_{-0.02}$\,deg$^{-2}$. The integration of the source count distribution yields that point sources contribute 16$(\pm1.8)$\,\% ($\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.
Comments: Version replaced to match the published one. Contact authors: M. Ajello and A. Tramacere