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

The Astrophysical Journal (Impact Factor: 5.99). 11/2010; 723(2):1082-1096. DOI: 10.1088/0004-637X/723/2/1082


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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Available from: Elisabetta Cavazzuti
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    • "Put simply, VHE γ-rays that would normally not arrive from distant AGNs or GRBs could become detectable because Lorentz violation would suppress the pairproduction of electrons and positrons that would otherwise have stopped them from getting through. VHE γ-rays traveling cosmological distances encounter low-energy photons belonging to the cosmic microwave background (CMB) and the extragalactic background light (EBL) due to starlight, both primary and re-emitted after absorption by dust [13] [14] [15]. At sufficiently high energies, according to conventional quantum electrodynamics (QED) some fraction of the emitted γ-rays are absorbed as they scatter off the EBL through pair production of an electron and a positron: γ V HE +γ EBL → e + +e − [16]. "
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    • "This energy range is of great importance for a wide range of scientific topics: the Universe goes from being transparent to gamma-rays to having a pronounced horizon at a redshift < 1 [1] [2] [3], limiting the number of bright very distant objects (such as e.g. Gamma Ray Bursts, or GRBs) that can be studied, but providing information on the cosmological evolution of the infrared-ultraviolet background light [4] [5] [6] [7] [8] [9]. the brightest Fermi-LAT sources [10] have spectra that steepen in this energy range. Examples for this behavior are flat-spectrum radio quasars [11], and mid-aged supernova remnants interacting with molecular clouds [12] [13] the diffuse Galactic background has a steeper spectrum than most detected Fermi-LAT sources and therefore goes from being the dominant c-ray emitter below this energy range (at $ 100 MeV) to being sub-dominant to individual sources in the TeV band [14] [15] [16] [17] [18]. "
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