The Astrophysical Journal (Impact Factor: 6.73). 01/2010; 723:1082-1096. DOI: 10.1088/0004-637X/723/2/1082

ABSTRACT The extragalactic background light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the γ-ray flux of extragalactic sources such as blazars and gamma-ray bursts (GRBs). The Large Area Telescope on board Fermi detects a sample of γ-ray blazars with redshift up to z ~ 3, and GRBs with redshift up to z ~ 4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of γ-ray flux attenuation by the EBL. We place upper limits on the γ-ray opacity of the universe at various energies and redshifts and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. can be ruled out with high confidence.

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    ABSTRACT: The emission mechanism of TeV blazar 3C 66A is studied employing multi-wavelength observational data. The GeV spectrum observed by \textit{Fermi}-LAT cannot be explained by pure synchrotron self-Compton (SSC) radiation. A multi-component leptonic jet model including the absorption of extragalactic background light reproduces the quasi-simultaneous spectral energy distribution. The external Compton component of the broad line region (BLR) is important for emission above 10 GeV, while emission from 0.1 GeV to 10 GeV is dominated by the SSC component. Our model implies that the redshift of 3C 66A should be between 0.15 and 0.31, the most likely value being 0.21. Our results also indicate that the $\gamma$-ray emission region is beyond the inner zone of BLR. The effects of model parameters and assumptions on the estimating of the redshift are discussed.

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