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We present a catalog of high-energy gamma-ray sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi), during the first 11 months of the science phase of the mission, which began on 2008 August 4. The First Fermi-LAT catalog (1FGL) contains 1451 sources detected and characterized in the 100 MeV to 100 GeV range. Source detection was based on the average flux over the 11 month period, and the threshold likelihood Test Statistic is 25, corresponding to a significance of just over 4σ. The 1FGL catalog includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and power-law spectral fits as well as flux measurements in five energy bands for each source. In addition, monthly light curves are provided. Using a protocol defined before launch we have tested for several populations of gamma-ray sources among the sources in the catalog. For individual LAT-detected sources we provide firm identifications or plausible associations with sources in other astronomical catalogs. Identifications are based on correlated variability with counterparts at other wavelengths, or on spin or orbital periodicity. For the catalogs and association criteria that we have selected, 630 of the sources are unassociated. Care was taken to characterize the sensitivity of the results to the model of interstellar diffuse gamma-ray emission used to model the bright foreground, with the result that 161 sources at low Galactic latitudes and toward bright local interstellar clouds are flagged as having properties that are strongly dependent on the model or as potentially being due to incorrectly modeled structure in the Galactic diffuse emission.

We have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/γ-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these γ-ray sources is similar to that of blazars discovered at other ABDO ET AL. Vol. 716 wavelengths, clearly showing, in the usual log ν-log ν F ν representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low-and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, α ro , and optical to X-ray, α ox , spectral slopes) and from the γ-ray spectral index. Our data show that the synchrotron peak frequency (ν S peak) is positioned between 10 12.5 and 10 14.5 Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 10 13 and 10 17 Hz in featureless BL Lacertae objects. We find that the γ-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter γ-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum γ-ray sources, the correlation between ν S peak and γ-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.

We report on the first Fermi Large Area Telescope (LAT) measurements of the so-called "extragalactic" diffuse γ-ray emission (EGB). This component of the diffuse γ-ray emission is generally considered to have an isotropic or nearly isotropic distribution on the sky with diverse contributions discussed in the literature. The derivation of the EGB is based on detailed modelling of the bright foreground diffuse Galactic γ-ray emission (DGE), the detected LAT sources and the solar γ-ray emission. We find the spectrum of the EGB is consistent with a power law with differential spectral index γ = 2.41 ± 0.05 and intensity, I(> 100 MeV) = (1.03 ± 0.17) × 10 −5 cm −2 s −1 sr −1 , where the error is systematics dominated. Our EGB spectrum is featureless, less intense, and softer than that derived from EGRET data. PACS numbers: 95.30.Cq,95.55.Ka,95.85.Pw,96.50.sb,98.70.Sa Introduction: The high-energy diffuse γ-ray emission is dominated by γ-rays produced by cosmic rays (CR) interacting with the Galactic interstellar gas and radiation fields, the so-called diffuse Galactic emission (DGE). A much fainter component, commonly designated as "extragalactic γ-ray background" (EGB), was first detected against the bright DGE foreground by the SAS-2 satellite [1] and later confirmed by analysis of the EGRET data [2]. The EGB by definition has an isotropic sky distribution and is considered by many to be the superposition of contributions from unresolved extragalactic sources including active galactic nuclei, starburst galaxies and γ-ray bursts ([3] and references therein) and truly-diffuse emission processes. These diffuse processes include the possible signatures of large-scale structure formation [4], emission produced by the interactions of 3 ultra-high-energy CRs with relic photons [5], the annihilation or decay of dark matter, and many other processes (e.g., [3] and references therein). However, the diffuse γ-ray emission from inverse Compton (IC) scattering by an extended Galactic halo of CR electrons could also be attributed to such a component if the size of the halo is large enough (i.e., ∼ 25 kpc) [6]. In addition, γ-ray emission from CRs interacting in populations of small solar system bodies [7] and the all-sky contribution of IC scattering of solar photons with local CRs can provide contributions [8][9][10]. Hence, an extragalactic origin for such a component is not clear, even though we will use the abbreviation 'EGB' throughout this paper.In this paper, we present analysis and first results for the EGB derived from the Fermi Large Area Telescope (LAT) [11] data. Our analysis uses data from the initial 10 months of the science phase of the mission. Essential to this study is an event-level data selection with a higher level of background rejection than the standard LAT data selections, and improvements to the instrument simulation. These have been made following extensive on-orbit studies of the LAT performance and of charged particle backgrounds. Together, these improvements over the pre-launch modelling and bac...

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