Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe

The Extragalactic Background Light(EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rayswith energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the gamma-rayflux of extragalactic sources such as blazarsand Gamma-Ray Bursts(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray blazarswith redshiftup to z~3, and GRBs with redshiftup 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 gamma-rayflux attenuation by the EBL. We place upper limits on the gamma-rayopacity 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. (2006) can be ruled out with high confidence.